I want this too. It could be a major help in a gym battle, for example, with proper timing, and it would just be an overall badass thing to see mid-battle.
You're kicking some Lass's butt in a battle and your Combusken evolves into Blaziken before she sends out her best mon, which happens to be weak to Blaze Kick. It's not a huge deal but it's still epic.
The only thing that I do feel should be a button similar to mega evolution in battle is Giratina's form change using Griesious Orb. It would be really cool if Giratina could change form mid battle without wasting a turn. And why stop at Giratina, Shaymin and Meloetta needs to be able change form in a similar way.