u can but think about this bondo is a filler that becomes hard and not flexable right
and a bumper is pretty flexable
those two together dont mix one bump at idle speed can break that bondo and end up lookin like crap
just sayn better to do it right the first time than to waste money in paint, materials, and time if u get the slightest bump
You are right, but you are not going to replace a bumper after every bump at idle. Bondo is after all, a plastic filler. If you have minor damage to a bumper, I see no reason not to fix it with some kind of filler. This is how they fix most bumpers at most shops anyways.
You can always use flexible epoxy if you are worried about flexability. Regardless, after a hit you will either need to fix a bumper or replace it. Whatever is in your budget.