It's dangerous because you're low, slow, with a big piece of drag behind you, not to mention many of the towing operations use aircraft that are long overdue for serious maintence. A good friend of mine spent the summer towing banners, and he felt lucky to leave with his life. He had engine trouble on numerous occasions, lost an engine once, and said the aircraft was held together with duck tape. I'm sure it'll put hair on your chest, but it still doesn't compare to instructing, in which you learn the info yourself better than you ever could have without teaching it to someone.