The way I see that this could work is similar to heirlooms: Your character could create a "spellbook" heirloom of their mounts (or vanity pets) that they send to alts whom can then use it to browse and learn the mount spells they want to have - if they so choose!
I'm currently rep grinding Cenarion Expedition for an alt and while I do not mind it too much (gets me a nice extra penny and some of the instances have nice things for mogging) it would be better for me if I could just grab the mount from my main. Personally I don't see how it's taking anything away from anyone if they can "juggle mounts" inside their own account. You worked for it once already, your alts shouldn't feel like they're missing out!
I do admit that when I saw the blue proto drake drop for me a second time; it was mind blowingly awesome. Still, it was already mind blowingly awesome to see it drop the first time and I would be more than happy to just "send" or "copy" the mount over to my death knight too instead of having to grind Skadi all over again.
I can see where the people on both sides are coming from with this, but I also believe there are ways to keep everyone happy even if mounts become available account-wide. I do not think it will be a blatant copy-over from one character that has lots of mounts to all others you have. Never say never but I'd wait and see before having any heartache to one way or another?
