Why not have it in the game though? If people don't buy single levels, then they don't buy it. Again, its only an option to give to players.
I know I would take advantage of that option, but again, not all the time. If im leveling an alt and i get to 59 and I just want to get right into starting the end game grind instead of grinding out another level, its my prerogative to buy a level boost. At least thats how i look at it.