So again I ask, does the lack of tri-spec add anything positive to your gameplay experience? I mean, do you enjoy running back to the trainer for that when everything else is already in the interface? If you do, that's great, but I doubt you are in the majority, and if not, then how can you not be for this?
Really the only semi-valid argument against it would be that Blizz would have to use slightly more storage to hold each character's data, but even then that kind of extra data would only encompass a handful of bytes, nothing sizable enough to make a significant impact even with the size of WoW's user base. Especially not with how cheap mass storage is now.
Also I can't help but laugh at all the "herp derp this will make the game more for casuals" posts, a small QOL improvement does not equal the game becoming casual, and I really doubt most hardcore players consider the class trainer their defining aspect.