I was under the impression that on the switch-in, the pokemon is "normal" and it can only mega evolve after you have committed to using an attack that turn (i.e. taking up one turn)?Mevolving does not use a turn to evolve.
My mistake, I completely forgot about that.MEvos can't use Choice items, or any items beyond their required Mega Stones for that matter. Why do people seem to be constantly forgetting that? :/
That said, the ability to hold a different item is likely the only factor saving a number of Pokémon from being completely overshadowed by their MEvos.
I'd say the main thing that stops pokemon getting overshadowed is the limitation of 1 MEvo per team. Unless it's a gimmick, we are likely going to look for a significant increase in effectiveness when we give up that slot. So in the case of Mega Charizard Y, although drought is good, is it really worth giving up the only mega stone when a Ninetails switch could also give you perma-sun? I don't know, it's just speculation of course.