That's why I think the whole mechanic is VERY interesting, and the main reason for including Megalutions was Eviolite.
This is the perfect Anti-Eviolite way to give new stages to our favourite Pokemon. Also with Charizard (and possibly few other Pokemon too) you gain additional twist, because the opponent is not sure, which Mega do you posses. I can see many uses for both. Can't wait for the second Venusaur, though.
Edit: That's a VERY good question. Would you even be able to switch a Mega, or is it to epic to be contained?