If I understand this right - you don't pick the CR of the elementals, you pick their maximum CR. You don't pick where it actually appears, it just appears somewhere within 90 feet of you. You don't even pick the element of what you are summoning.
To summarize, you can cast Conjure Minor Elementals at 8th level in an underwater fight, select the option to 3 elementals of CR 2 or lower, and receive 3 CR 1/2 Magma Mephits that are 90 feet away from the fight? In character I would know I am not picking any part of the what I am summoning, I'm basically just yelling help into the void. Why would I waste a 4th level spell to summon a completely random something instead of using something like Polymorph, which just gives you the creature you want and does so by giving something a massive bonus health pool?
Is there any use for this spell besides the tiny benefit of just a couple extra bodies with at most like 40 health - which you can get 5 of with animate objects?