I have one of each class at 90 already. I'm not completely sure what I'm going to use the free one for at this point.
Sitting on 18 90's and a few close to it no need to buy them leveling is a joke
Depends entirely how you value your time. If you get paid $7.45/hr and the cost of WoW is BattleChest (20), MoP (30), and WoD (40) we can determine it would be $90 to get a free level 90, meaning you would have to work 12.1 hours to break even. It takes AT LEAST 15 hours gametime just to ding 90 with heirlooms and Darkmoon Faire buff.
It'd probably be worth it for most people, especially since "powerleveling" costs hundreds.