They tried that in Cata, where doing a rep just meant dungeon running. And fun fact, Cata was stupid in that you spent all of your time in Stormwind. It doesn't take all that many days of dailies to finish off a faction - I only resubbed in December, I leveled 2 different chars to 90, and now my main is exalted with golden lotus/klaxxi and shado-pan and celestials are coming along quickly. Dailies get you out in the world, and there is definitely something to that.