A corollary question might be 'why wear full gear when it makes the EMT's job (or doctor's job) that much more difficult to get to me after an accident?'. These guys not only have training, but they have tools (sharp scissors, etc.) that will make short work of jammed zippers, armor, etc.
Last time I got picked up by an ambulance... er... only time... while riding and crashing the EMT's did a heck of an outstanding job to remove my protective gear (leather) without damaging a thing. I was just waiting for the nice sharp and powerful scissors to go right up my sleeve and cut the leather out of the way. But nope.. they took the care and time to get my jacket partly off me so they could measure bp and start an IV. It wasn't an easy effort and I am very happy they did (since that jacket is no longer made). The ER got the pant's off in one piece too.
I asked them about it later and they said it was simple, the jacket is apparently a good one, it did it's job and made their job ultimately easier. It had some scuffs but no holes or tears, so very little bandage work was needed to be done by the EMT's to me, that meant no scrubbing the back of the rig from all the blood that would be leaking out. They know if they cut it off it is destroyed, and they are not cheap. In the following weeks I may go for another ride and I might not have the money to buy another jacket. So if they can get it off in one piece they know I will be wearing it the next time I am out and might need their services... and then hopefully they won't have to scrub my blood off of anything, again.