I sometimes get taken to task by reality folks when I talk about preparation for training. I switch to my old glasses, tape up my hands, sometimes put on a knee brace or two, and if we are getting fairly active, wear a mouthpiece. I also warm up, stretch, and wear an old T-shirt and jeans or shorts, weather permitting. And we don't use real knives, either.
You won't get to do any of that in a streetfight, the hardcore reality guys sometimes say. You have to be ready to go as you stand, badda boom!
Absolutely true. If I have to run home and put on my gi and do ten minutes of stretching and warm-up, I probably don't have a useful fighting art, unless the duels are arranged.
Then again, while there might be a lot of energy expended in a short time, a streetfight isn't going to last long. What you do there in ten or fifteen seconds isn't apt to damage you nearly as much (assuming you know what you are doing) as a year's worth of hour-and-a-half training sessions will, vis a vis your joints, muscles, and such.
The old ideal two-hit fight -- I hit you, you hit the ground -- will be over in a few heartbeats. And chances are, if you are paying attention and trying to avoid such encounters, you won't have too many of them. Whereas if you train regularly, you will expose yourself to strains, bruises, breaks, contusions, and the like. You'll also do damage to your clothes, and having to replace your eyeglasses because a punch slipped through and broke 'em gets spendy in a hurry. I speak from experience.
So you prepare for the "reality "of training. It might not be a real fight, but chances are that you are more likely to get hurt training than fighting, and using some basic precautions can spare you some of that.