Really? Cause on most teams I've been on, when aggro-holding is discussed, it's usually a tanker or brute that they look to. I've not seen too man even think about scrappers when they are talking about holding aggro. Especially not in high level content. Example: Hamidon Raids on most of the servers, the taunt team is still basically tanks, unless I've really missed something.
EDIT: Also if a scrapper doesn't take anything to hold aggro no one bats an eye. A tank (and sometimes a brute) not taking any aggo holding powers like Taunt, and the sky is suddenly falling.