When I was a kid I was into hot rods. I had a lot of friends who worked on cars and did customizing. There was a very strong separation between the guys who actually formed their metal parts and those who used Bondo. Bondo was looked down on and considered to be an inferior shortcut.
Later, when I began my foray into woodworking, I often had the need to fill imperfections in work that was to be painted or veneered. I tried all of the well known "tricks" like mixing sawdust and glue, using epoxy or gluing in "dutchmen" to patch less than perfect areas. I don't really remember exactly when I remembered about Bondo but at some point it occurred to me that this might be a good filler for patching and filling paint grade work or for veneer substrates.
Once I tried it, I realized that, while this might be considered an "inferior shortcut" for forming a wheel well flare, it was the best wood filler I ever used. Since then, I have never been without a can of the stuff. Of course, you can't use Bondo on anything that will have a natural or stained finish. But then again, we shouldn't need fillers on such work anyway. At least that's what I've been told