My wizard does have Conjure Minor Elementals, and it is one of the better options for its spell level. The main reason is that what it does is so good when you select the lowest CR tier, it does not really matter what exact kind of elemental you get. You get 8 different creatures that will stick around for an hour of concentration, and that can soak up attacks, get in the way, scout ahead to trigger traps, make object interactions or carry stuff for you, nevermind attacking, and that even can be sent off to attack opponents far away from the safety of your inn or tower. Due to the action economy, that's very useful, no matter what the other details are. You turn your one action per turn into nine. There is no lower level spell that can do that for you.1
In the core rules, the only elementals you can get at CR 1/4 or below are mephits, of the Mud, Smoke, or Steam variety. They are all able to fly, all have more than 20 hit points, all have at least 5 strength, and all have human level intelligence, so you can give them complex tasks that they can carry out themselves. If you play using the core rules, there is not really a lot of variability. Some of them are a little stronger, some a little faster, Smoke ones are great for scouting, Steam ones are great against melee opponents due to their ranged breath weapon, but really any of them will do a servicable job.
If you take all published sources, there are a few other options. If your DM elects to give you Chingwas you'll get elementals that do not have attacks, for example. But even then this will be quite a useful spell, you will get 8 different uses of supernatural charms out of it, and they still can get in the way and soak up attacks. Any single attack that does not hit you but hits one of them instead is in effect like a healing spell. It is possible that the spell will get even weaker over time as more and more options are published, so the DM can pick the maximally useless one for any given situation.
However, not all DMs are out there to screw you over, and they should't be. In all campaigns I have played in, the DM has been pretty willing to conjure either the exact type of elemental the player requested, or at least one that was a fit to the environment. With this being the table norm, the spell becomes even better than it already is.
Even if your DM decides that in an adventure where you fight salamanders above a river of lava, Steam mephits or some other fire-based elementals are appropriate, and the extra fire damage from them is useless, that is not so bad: they still can deal normal slashing damage, still can get in the way and block or sponge up attacks, and they even have immunity to fire themselves, making the fire damage the opposing monsters deal useless, too.
It's nice if a spell does exactly what you want, but it is not a neccesary condition for the spell to be good. As long as the effect is powerful, and broadly what you want, the spell can be good. Imagine you had a spell that summoned a random demon lord to do your bidding. Would it do exactly what you want? No, if you want Orcus, you might get Demogorgon instead. Would it be super powerful? You bet. Except for extremely unusual circumstances, any one of them would be an insane tool of destruction for you.
To give you a couple concrete situations where the spell was better for us than any of my wizards other, lower level and more specific options:
- we were trying to get treasure (piles of coins) from an island in a lake in undermountain, and we knew an Aboleth was in that lake along with a bunch of other submerged monsters, able to dominate and take over any one of us who would have tried to reach the island. So none of us wanted to take the risk and fly over there, or spider climb along the ceiling, or misty step there (if the distance would have allowed that; I don't recall) or whatever. Instead we summoned 8 expendable mephits and sent them off to retrieve treasure: losing a few them to the Aboleth's nefarious powers did not matter one bit to us. They could fly there and fetch the treasure, and we did not really care that much what kind of mephit we got.
- we were inflitrating Xanathar's lair. We summoned 8 mephits, and sent them off to create all kinds of distractions and engage with guards to block our escape. No other single spell of lower level would have done that (my wizard also has animate dead, but these would have needed a lot more micro-managing than the mephits which we did not have time for, and we would have needed to smuggle in the raw corpses, which conjure minor elementals did not need). Again, the exact type of mephit did not matter that much.
So in general, the spell is especially good in situations where opponents have no immunities or resistances to any of the elements, when you need expendable helpers that test the wates of a dangerous situation you would not want to go into yourself (have your familiar ride along to observe first hand what happens), or when you just need a lot of bodies to gum up the works in battle.
1Lower level minion spells
Well, there are a few lower level spells that give a wizard a kind of minion, and most are deterministic, but all of them fall short in some way:
Summon Lesser Demons from Xanathar's that can summon 8 CR 1/4 or lower demons, and is level 3 instead of 4, but these demons will attack anyone, including you and your team, they will not follow your orders. That's a big difference, as it does not really give you additional actions to command, instead causing general mayhem and destruction. (Plus, this one also gives you a DM-selected type of lesser demon, so is not deterministic).
The above mentioned animate dead can give you up to 4 minions when cast with a level 3 slot, even with 24h duration and needs no concentration. It's also quite good. For me the most detrimental aspect is that necromancy is at best unsavory, and at worst (depending on the campaign) utterly evil and will get you burned at the stake. You need corpses for it, you only get 4 instead of 8, and the undead are pretty dumb so need constant handholding, and if you can rely on getting some kind of mephit, they lack flight.
Unseen servant can do some of the tasks like triggering traps, but cannot block, fight, think, and you only get one of them, not 8. It is also limited to close range. The elementals are able to travel a long way in the hour you have them to get somewhere you want them to be.
Find familiar, arguably one of the best wizard spells in the book for what it does for the cost can get you a single helper, and you can use its senses on close range and it can deliver touch spells. But it cannot attack, has a gp cost, and again you can only have one. (In addition to the elementals. This is not really a question of either or - you take both the familiar and conjure the elementals).
In a way, mental manipulation like suggestion or charm person might also net you an ally. The downside here (apart from again getting only 1) is that those have issues with dying for you — and unless your wizard is an evil one, so should he or she. The elementals in comparison don't pose any moral problems. They are expendable. They just go back to their home plane when they bite the dust.
Other lower level spells
It is a bit apples to oranges, but you also can compare the spell other lower level spells, that may be more useful for a specific situation, for example you can use the fly spell to fly instead of having a bunch of mephits carry you along flying, you can use knock to open a locked door, instead of having them bash it open, you can use fireball to fry a group of wolves, instead of having them boil them up with steam breath, you can use mage hand to open a possibly trapped chest, instead of sending an elemental to do it, you can use clairvoyance to find out if someone is in the castle couryard instead of having a mephit fly to the crennelations and report back, you can use glyph of warding to guard a corridor behind you instead of them blocking it, etc., etc. The key point here is that having 8 durable helpers is extremely versatile, in a way many of these other spells are not (and all of them are solid or even great spells). For the cost of just one preparation slot, you get to deal with many different situations, and free up preparation slots for spells with unique abilities.
P.S. The larger downside in my experience for this spell is the 1 minute casting time. I wouldn't really care what 8 combatants I can get, as long as I could get them for a 1 action casting time, and bring them in in a fight. As is, the spell is only really useful for combat, if you know it will be coming, and you can cast it in advance.
P.P.S. Because adding so many additional creatures is so powerful and is also hard to manage at the table, more recent conjuration/summoning spells have moved away from this template, and instead focus on giving you a single creature that gets stronger along with the caster, like the Summon Elemental spell in Tasha's.