I, for one, am for in-battle evolutions. As Samurott said, sometimes my pokemon level up twice in one battle, miss their evolution level and therefore don't learn a particular move. While you can relearn it, if they put the relearner in the last city before the league (I'm talking to you, HGSS), then it's likely going to be a while before that opportunity comes around. Sure, I could take it out of battle and not use it again until the battle is over, but if it's the one pokemon I have with a Super-Effective move against my foe, then I'm going to use it. I think it would add a really interesting dynamic to a pokemon battle, and to be honest I'd probably try to time it so that each of my pokemon evolved deliberately during a battle. I won't be disappointed if they don't do it (hey, I've survived the last 5 gens), but I think it would be cool if they did.