Actually, it IS a pain in the ass. Take this scenario for example:
Two keyboard users are going to battle each other, both of them just happen to have the -exact same- configuration, as that is what they are used to playing, what are you to do now?
Flip a coin to see who keeps it? What if there is only one keyboard(althought probably they would each have their own).
Are you going to be monitoring where they press every single key?
All of this is too much of a hassle, it's just way better to not allow them at all. Now, I am assuming you can configure keys for keyboard in MBACVerB, I remember you couldn't in MBR(I think), I am not so sure since I've never played on the keyboard, but still, that situation can show up, can't it?
I would say keyboards are a definite no.