True, but I think a DM (any DM, not just a good) can work in opportunities for any PC concept that's viable in the setting. Otherwise the AP just becomes a railroad.
You may be right. For one thing, I suspect that, left to their own devices, people try to play to their strengths. A mounted fighter will gravitate to areas and adventures that use his or her skills.
The problem is, you've also got three or four other players at the table.
Three players are perfectly happy doing dungeon crawls, sailing on ships or going to otherwise horse-unfriendly places because their character concepts don't include mounts. But, that one guy is now pretty much screwed because either he's happy and the other three aren't, or they're happy and he's not.
There's always the issue of the rest of the group as well as just the player-DM axis. When Mr. Cavalry Guy wants wide open spaces, Mr. Dwarf wants dungeons, Mr. Elf wants forests and Mr Pirate wants a ship, SOMEONE'S going to be unhappy all the time.
Sure, it's fine to say, well, it's shine time, everyone get's their spotlight, but, in the above situation, one guy's only happy about 25% of the time and spends 75% of the time in positions that suck.
The Paladin's Mount thing has ALWAYS been a problem. I don't know how to solve it either. Mounts are just so limiting. It's not just dungeons - any urban environment, jungles/heavy forests, anything on water (or under water for that matter), mountains - the list goes on. A mount centered character is a really hard archetype to pull off because of this.
Note, I did see mentioned the small rider with a medium mount and I have used this to great effect. It's limiting, but, at least you actually get to use your mount instead of writing off the whole thing.