Who knows, maybe his Special Attack will be high enough to use stuff like Dragon Pulse effectively. Plus, if the Mega Stones require some sort of activation, it'll be hard to keep switching out Megampharos because of the SpAtk drop.I just noticed a probable issue with Mega Ampharos.
If it can only change in-battle, then it won't be able to learn Draco Meteor assuming the same mechanics to learning it still apply.