Mega evos are temporary, likely won't be seen outside of battle much or at all, are limited to a select few pokemon, and some can't even be accessed without an event to give you their megastone. I have a feeling we won't be seeing much of mega evos, even though they're rad as hell and an interesting idea.
They're also going to pose balancing issues; GF either balances the game around them and teams without a mega evolution are going to have trouble, or they don't and you can just stomp through every single enemy in the game with a mega evolution with no difficulty.
