i think if you train with a pretty bad injury, and fight with one, then you have only yourself to blame. certainly, doing so makes one exempt from using that injury as an excuse, like so many fighters do. "oh, i'm not using it as an excuse, but i'm just sayin'..."
and if you do train or fight injured, how long do you think that will benefit you? if you keep healthy and train well, you can fight for a long long long time. no need to stress your body now and retire when your 35.
be smart and train your ass off when you are healthy, not when your injured. training isn't the same as physical therapy!