Evolution of the Nuclear Type

[Polaris]

With the present Mega Evolution mechanics, making it unlimited would break the metagame. If you think about all the strategies available in pokemon, most are optional. However, Mega Evolution's so powerful (most megas end up stronger than pseudo-legendaries, and it doesn't waste an attacking turn) that no one really runs a competitive team without a mega on it. Allowing multiple uses per battle would make mega-capable pokemon into a large fraction of the meta - instead of running one per team, you might see 3+ slots devoted to the pokemon that get mega evolutions. Limiting it forces players to make choices and brings in player reading - you have to guess whether that pokemon will be their mega or if they have another one and are just running a normal set on this one. When the answer to that question becomes "all of them" the guesswork disappears. So while running multiple megas might be fun, you'll find it becomes a lot less fun when everyone is doing it.