I'm not one that complains a lot about things blizzard implement, I typically can see good reason for most things. Not letting your alts use mounts you've rep grinded for is extremely silly to me. Every other mount works, why doesn't this one? I wouldn't have done the rep grind and gotten the mount to begin with if I knew it wouldn't work on all of my other toons (without rep grinding). I understand it's my fault for not looking too deeply into it, but still. They should consider changing this to an account wide passive once your mail learns it, or take the skill out entirely imo.
They probably will eventually. I would guess that they just didn't want the cloud serpents to be super common right away.
I agree with not wanting them common right away, I'd have taken a 2x longer grind in exchange. I wonder if they plan on removing it with the rep changes. I wish more types of mounts were like this to some extent. For example you had to go through some training process to learn how to ride yaks, just like the serpent mounts require you to go through this. I just wish it was account wide.