
You said there was pressure to send character specific mounts to paladins because their 3 extra class mounts allow them to have a higher total. But when you are 3 mounts away from the 200 achievement, there are more than 30 other mounts available in game, from various sources. There's no reason to feel pressured because there are plenty of other options. Those who feel pressured are making a logical error. They weigh the class mounts (and charger) against the other higher tier mounts they are missing, when in fact they should weigh a CM Silver and two sets of 100 3v3 wins against those missing mounts, because this is what they are either a) taking away from their mains or b) doing a second time for the sake of the paladin.
Of course, there are those who decided they would main a paladin from the start due to the extra mounts, but I don't call that pressure, I call it choice and class perk. And there are those who do arenas and CMs on a lot of chars, and naturally some of the mounts may end up on a paladin, but there's no problem here, like you initially implied.
It's hard to argue "objectively" why filling your collection with mounts you can't use is bad. To me, it feels like padding. They are there, they "count", but they don't really matter because the only thing you can do is look at them. But, of course, I feel comfortable enough with my collection count atm. Maybe for others this padding feels great because it brings them closer to a reward they thought was very far away, so they subjectively value it higher than I would.
It's also important to note that at any given time I'm playing my "characters", not my account. I think it's sort of cool that my paladin has his own unique mounts that my mage does not, and I see no appeal in my mage seeing them in his journal. You said other people might feel differently, but failed to explain why.
Mind you, it doesn't bother me if the mount journal could display all your mounts if you asked it to. I can see the point of that, since they now have their informative segment much like the pet tab (although it's not as necessary as it is for pets, which you can potentially encounter in pvp). Just as long as a) they have a filter, Horde/Alliance or Character/Account, so that I can opt to only brows through the mounts I can use, and b) the count for the meta achievement is character specific and only counts mounts that can be used that particular char.