you have no means of knowing if you are over tightening the bolt or if it is not tight enough as well as most spindle houseings are aluminium.
take a piece of cast aluminium and hit it with a hammer hard and see what happens or even a peice of cast iron, it will break.
to some extent that is how a impact gun operates and the excessive tightness and hundreds of impacts per second will give you the same result.
even car and truck tires are supposed to be torqued by hand to a specific ft lb of tightness, so why would you do that to a little rinky dink spindle?
if everyone were jumping off of a cliff would that mean you will follow.
just because everyone does something does not mean it is right, i think the reason why you see so many people doing it is because they are lazy, afterall we live in the society of lazyness and everyone wants the quickest easiest way to do things.
depending on the size of your mower what will you have, 1 to 4 bolts to remove by hand? if you put ant sieze on the bolts prior to replaceing them which you are supposed to do anyway, you will never have to fight a bolt again.
you can't expect power equipment to make up for the improper techniques applied during maintenence regimines