I'm not a fan of this idea. I think it would distract you away from the main object of the game; defeating your opponent. Farming the opposing factions creeps for a level advantage provides two incentives - your fighting your enemy, possibly pushing further down your lane and your getting gold/exp as you do so.
This idea - as it plays out in DotA - leads people to ignore their lanes for chuncks of the game at a time to farm these neutral units so that they can overpower their opponents as early as possible. I'd prefer the game didn't devole down this path. Personal opinion however.