It's far from similar. There is a big difference between a short period of boosted XP that runs out before you return to normal, and your XP grinding to a complete halt after playing too much. Ashthra's post pretty much hit everything on the head. I think originally you had to be logged off of WoW for a week to get an entire level's worth of bonus XP, so day-to-day rested XP will be a nice little boost while people that can only play on weekends or every other day get a bit of a much-needed boost to keep their pace up.