I can understand why you feel that way, but not everyone cares that much about RP factors. There's nothing wrong with someone using a charger mount on a warlock, nor using a HH mount on a paladin. I do agree that class specific mounts should stay class specific though.
That's really a poor argument. You're essentially saying that RP factors shouldn't inhibit player options and gameplay, because not all players care about it. But the reverse argument is just as valid--that player options and gameplay shouldn't inhibit RP factors.
Ultimately, WOW is an RPG. And every decision needs to be balanced around the various needs of a role playing game. The game world is created by the hundreds of small details, not just the grandiose ones. Because, generally speaking, the grandiose constructs are the ones you can most easily compare to other games. What makes Stormwind Stormwind and not some random, human Medieval-style city? It's all those small details--that kid chasing his sister,
, the Mage Quarter (complete with Warlock pub), the tailors shop on the water, etc. When you play multiple games with similar cities, those small details are what keep them from becoming the same thing in your mind.
Of course, that's an example from the world environment, but the same thing applies. You create classes to be distinct because they're supposed to represent fundamentally different styles of life. The reason your Paladin can't ride a Dreadsteed is because he's a Paladin who abhors anything that does not serve the Light, which that mount most certainly doesn't. The more you gloss over these lines, the less distinct the classes become. The less distinct they become, the less interesting they become. And, ultimately, the more generic they feel.
Take away the fluff that makes a Warlock feel like a Warlock, and he just feels like "generic shadowy mage archetype".
Yes, we're just talking about one system here, but limited quantity is never a good argument--you can apply it to anything, and eventually you've made one too many changes on the grounds that "just one won't make a difference."
So, no, not caring about RP factors doesn't magically make it okay to ignore RP factors, because your play experience there affects the experience of everyone. There's no meaningful increase to the non-RP player by allowing class-specific mounts to be cross-class, but there is a meaningful decrease in enjoyment to the players that do care about the RP.
What makes it even worse is that all the one side would be gaining is aesthetic options, where the other side would be losing something that's part of the fundamental fabric of why they enjoy the game. You see your Paladin on a Dreadsteed, laugh for a moment, then go on your merry way. They see your Paladin on a Dreadsteed and feel the crumbling of the lore that created the world they're enjoying.