I learned the Over The Head method of putting on a backpack in the Army, where rucksacks are often too heavy to do the one handed jacket style donning method, and where you may be in way to much of a hurry to be able to wait for a buddy to hold it for you.
The OTH method seemed to naturally lend itself to diving. The gear is sort of heavy, the straps and buckles are easy to tangle, though they flow right into position OTH.
Could you get hurt doing this? Sure. It would be all too easy to strain a muscle. If you are standing up, you could lose your balance, especially on a boat.
If you are using a SS BP/W like me, and your hands are wet, you lose yor grip, dropping the tank on your head. Then you would fall, hitting your head on one of the standby tanks under the bench, knocking yourself out cold. Then the gear you were putting on comes crashing down and shears off the valve of the tank you hit your head on, causing it to shoot like a guided missle towards the transom, poking a big hole in it. This in turn will tick off the captain and crew, who are likely to throw your @$$ overboard. And of course you are still unconcious and quite probably bleeding. If you happened to be at one of those shark feeding dives, well, things could get ugly.
Even knowing all that, I still use OTH as often as I don't. Except at the aquarium I volunteer at, you get the boot for doing that, strike 1.
I can see why many people would not want to try it. I can understand how easy it is to hurt yourself. I can even see where it appears that I use this method to try and prove how tough I am. But honestly, I just do it because it is the single easiest and fastest method to get kitted up, and I have done it for more years than I care to remember...