Questing is going to be the primary form of xping, so why would you go out of your way to kill stuff that doesn't directly contribute to the quest you're working on? (quest mobs have historically had different names and are instanced, so this is my argument based on this information. could be subject to change in ARR but I doubt it) Yoshi-P has already stated that monster grinding won't be a very viable way to level up, but I guess if you want to go back to old school style FFXI where you grinded stuff for an hour to get a level, by all means.

I think just having to run around the whole area to open up your map is good enough. I believe Yoshi-P said there were going to be achievements for doing this as well, though I'd be surprised if there were any XP rewards tied to it.