Filler is should be used to smooth out minor imperfections. You have to knock the metal back to nearly there. I prefer the short strand fiberglass (kitty hair) over bondo for filling anything that is just a little less than perfect. It is much harder than bondo - doesn't marr or dent like bondo can. Can be built up a little thicker than bondo.
No excuse for not doing bodywork though. Either have to bump it with a hammer and dolly or cut it out and patch new in. At least not on a project that you want to be proud of. Once you get the kitty hair on, smooth, and right - then glaze over with the bondo to fill in the gaps. As was mentioned before, most of it will (or at least should) get sanded off anyway. Very few filler jobs are "one and done" and require multiple passes at apply, file, sand, repeat. And when you think it looks and feels right, lay some filler primer over the top, and be prepared to start the process over at least once more.
Also make sure that you are feathering out 4x to 5x the damaged area with the filler and using a long flexible board over the damaged area, rather than simply filling the dent/hole/patch. You will need to work on it to get it to blend into the entire panel.
