Well, a few things:
1. The battles will start to get repetetive.
2. What's the point in restricting it? Who cares if people are using jutsu that their characters can't use? It's RP, you're playing the role of someone who you can't normally play as, so if you're being so restricted during it, takes away the point don't you think? There's already enough restrictions on everything: Two bios, no of the same KG, etc. So now it would be like some characters can only use certain things.
3. People would start throwing around random customs, making battles even worse. It's like everyone just starts throwing around the same customs, would that be any better? Nope.
4. Also, due to the "no avoiding" jutsu rule, it's almost impossible for certain characters to dodge certain jutsu without the battles just becoming retarded. If you're playing as a ninja who has maybe one element and a KG or something, Naruto himself is a good example, how would you dodge certain jutsu? And then there's those who can use almost all elements, so if one who can use one or two elements fights a custom character or whatever who can use all... well... ya know? :|
