Burning trash is not so bad if you pick out the residue from the fire pit. It can reduce the weight of metal conatiners that you pack out. Most folks that burn trash, however, don't bother to clean up. As far as the harmful gases go, the are mostly harmful to the folks around the fire. So what goes around comes around!
I generally don't build a fire except at campgrounds (car camping). I also cary a trashbag and cleanup an old campsite on my way out from backpacking trip.