I think a lot of people don’t let their grill get hot enough. I know I have problems with cleaning my smoker racks (and my wife won’t let me run them through the dish washer) so I have to throw them on my charcoal grill when I’m heating it up.
Yeah, probably. Charred food is not good for you really, but it's going to be on there anyway after you cook on a grill no matter what (unless it's like, a George Foreman...?). I can't imagine the bit of carbon on the grill from the last session would ever cause more harm than normal charcoal!