I've seen it requested before, I was watching Wales v Italy in the u20's World Cup last year, and Dan Biggar kicked a long range drop goal which looked to go over, but the ref said it missed and called a 22 drop out. Biggar asked to go to the TMO, but wasn't allowed to because there wasn't a TMO. If there had been then Wales would have had the droop goal, because the coverage had a second look at it, and it went over. So I don't think it's that bad an idea since it's useful for accuracy of calls, but then again it does make you wonder if refs are too reliant on the TMO.