Yeah but say it levels up five times in the course of one battle, you'll have to press B five times, it gets annoying. And some people wouldn't want to make the pokemon's item a everstone.
What I meant is a Pokemon can't evolve inbattle if it had been denied to evolve once. So for example, my Magikarp gained a level and wanted to evolve, and I press B. This means no matter how many levels I gain in this battle he will not evolve because I already denied him.
I've been wanting this since R/S/E, it would really shake things up, change the flow of battle, and turn the tables in your favor at the last second, just like in the anime. I always thought a Pokemon should evolve during a battle if it happens to reach that level. Apparently the battle system is being tweaked, so this might be a new mechanic that's added.
This would be so neat, except for when you don't want to evolve something and you have to press the B button a ton of times. :( But at least you wouldn't accidentally gain too many levels and miss a move that you gain upon evolving at that level (Combusken's Double Kick at 16 after Torchic evolves, for example). I think it'd be worth dealing with the B button pressing, since I don't think it's often that people keep something unevolved for that long.