Uh....no. First of all, the paladin's mount is not a 'use it or lose it' ability, and while he certainly can't successfully take his mount down a '45 decline into the underdark or into a thieves guild with winding passageways, he CAN leave it at the stable of the nearest town after which he can retrieve it.
Likewise, a cleric can go weeks if not months without using his turn undead ability. In 3E, both classes only get to use their special abilities (or creatures) when the right conditions are present - paladins on relatively flat, clear land and clerics when undead are present.
And in my experience, adventuring parties find themselves on flat, clear land a LOT more often then they find themselves facing undead.
Sorry Zogg, but you have missed my point.
Part of what I said was "going somewhere where the mount can not be used either severly disrupts the flow of the game or permanently disables the paladin by removing a class feature"
Just casually stating that "he CAN leave it at a stable..." assumes a lot of flexibility. VERY frequently running back to town can be far closer to my "severe disruption" than it is to just something you do.
Next, you are treating the mount like any other horse. If I am to choose between a paladin saying:
A) This is my Holy Steed, gifted to me by the Lord of Dawn. He is a majestic, intelligent and faithful servant, mount and companion. Now excuse me while I go find what stable boy in this shanty can keep his oat bag full.
or
B) My Celestial Steed resides in the eternal grasses of the holy realm. He comes to my in times of need and serves me well. When my need passes, he returns to run with the winds of paradise.
I know which one sounds like a cool paladin to me.
Lastly, it is a far stretch to assume that dumping a mount "in the nearest town" makes that mount safe. (unless the DM metagames security, which instantly defeats the "pokeman is not believable" arguement). Once the mount is left behind, the paladin does not have it period. Eventually he can go back to get it. If it is still there. If it is lost, the paladin permanetly losses a class feature. (And a year of game time equal forever in many campaigns). I do not care if the party fights on 50 plains between every undead encounter. Turn Undead remains a class feature available whenever it is needed, with no concern that it may be lost. A lost mount is gone.