I have no problem if you want to eat your burgers raw, rare, or burnt; it's your body. But it's really not a matter of who's cooking the meat. People eat raw meat, like steak tartare, and it's safe because of how the meat was prepped. So I think if you trust your meat, cooking a burger to medium rare is fine. But if, like most people, you're buying your ground meat at a grocery store, it doesn't matter how good your cooking skills are, there's no way to know if that meat is safe.