I think what Jonathan wrote is true. I'm not certain the word necessary is in place but sometimes injuries are unavoidable.
The other thing is that maybe sometimes getting injured is 'good'', if you hope aikido will ever help you in a real live situation. Our sensei always tries to relate techniques to real life situations. Many times he said- there is a chance you will get hurt, but what is important is to continue and not close in on the pain.
I think that is true and important for any MA.
though I hope never to actually need to use aikido in a real fight of any sort (verbally I use it all the time), I think it's important to except the possibility of getting hurt and learning to deal with it.
"What happened to you?!"
"Forget how I look, you should have seen the other guy..."