Yes, it's worth it, even if you don't use it very often.
On the campaign level view, you won't necessarily always be dungeon delving, and you don't always have to have it prepared. Like a cleric, you can prepare it when you need it. But once prepared, you can always use it to fuel Divine Smite if your mount is alive, or if you don't need the mount for a while.
We found that summon steed was a good use of down time between adventures to summon the steed if the next adventure was where being mounted is useful.
Paladin Spell Casting
You can change your list of prepared spells when you finish a long
rest. Preparing a new list of paladin spells requires time spent in
prayer and meditation: at least 1 minute per spell level for each
spell on your list.
Our Experience With Our Paladin
What we found in our first campaign was that the dungeon delving intensive parts didn't often need a mounted (dwarf) paladin. When we got to a more wide open part of the campaign, in the Underdark with long journeys in high roofed caves, mounting the dwarf aided the party's tactical flexibility a lot, and allowed him to close quickly during combat. His mount was a DM homebrew that was not quite a giant boar, larger than a boar, and about the same stats as a warhorse but shaped more like a boar. (Our Dwarf loved it).
We also found that it was best to use the steed for movement, maybe one charge attack, and then the Paladin dismount while the steed withdrew. That kind of tactical detail will vary with your campaign and party, however. As our Paladin did not have the mounted combat feat, it ended up being better to keep the mount alive once initial contact was made.
If what you mostly use your spells for is Divine Smite (ours did, nova damage is nice) it doesn't matter what spell you have prepared. That second level spell is as good as any as Smite fuel. In some situations it may offer you a way to mount up and travel great distances when circumstances change or the rest of the party is on a short rest and you think you'll need the steed for ... something ... to include carrying loot!
Suit your prepared spells to what you're doing
Because you can vary what spells you have prepared, the question you have to ask yourself before going into a dungeon is: will I need a steed to complete this mission? If not, prepare something else, or leave that prepared "just in case" with an eye toward mostly using it as smite fuel.