I doubt there are many MMO's even offering level boost, maybe xp multipliers, but not level boosts.
actually their design decision has kept the levelling pace quite even throughout expansions, levelling from 1-90 is quite close if not much faster than levelling 1-60, they offer RAF opportunities that makes rush through 1-85, not skip, but rush, and until now they offered free 85 (?) for scroll of ressurection.
Though you are right, it's bad news adding this as a option can quickly turn into an excuse for not expanding RAF levelling to 90, and also an excuse for increasing the levelling speed through pandaria (e.g. reduce the xp)
As long as they keep evolving the present options it's ok to add such a feature, if they don't it's something that will cost blizzard a lot of subs.
Also this doesn't really concern me as I got all classes above 80, levelling the last 10 lvls isn't really an issue, cata goes quick and MoP is well not that long really and got alternate levelling methods