I'm in medical school, and work in a hospital in a major city. You would not believe how mangled a lot of patients are when they come in from being in motorcycle accidents.
There have been MANY studies done on protective gear, and they mostly agree: If wearing a DOT approved helmet, you are ~30% more likely to survive a high speed accident.
I'm convinced.
The amount of road rash between a t-shirt, 'regular' leather jacket, and armored motorcycle jacket is like night and day. It makes a HUGE difference. Wearing the jacket is just as important as it fitting properly, and being fastened properly. You would be surprised how fast the road will burn through a 'regular' leather jacket.
Again, I'm convinced after seeing as many motorcycle accident patients as I have.
I used to ride with a 3/4 helmet, jeans, and a 'regular' leather jacket....
Now I wear a SNELL full face, and armored pants and armored jacket. Do you know what the funniest thing is?? I'm COOLER (temp wise) in the summer wearing my armored motorcycle pants and armored jacket than I ever was in jeans and a leather jacket. The vents and mesh in quality protective gear are engineered very well, to the point where the 'proper' armored gear is actually much more comfortable and cooler than your 'normal' clothes. Go figure...