no reason not to do both, unless you have leveled a lot of chars questing is fun to see the lore and rewards and "flavor", but sometimes you get bored of questing and just want to kill a bunch of bears/pigs but even that gets boring (to me) after 30 minutes so its back to questing...

so many people try to rush to level 80 and then find there isn't much there that they actually enjoy (raiding, arenas, bgs, etc). sometimes leveling itself can just be a lot of fun.