I don't know why you worry about RMT spamming world chat when it makes it easy to point fingers at who exactly is a RMT and god willing once SE starts getting on RMT like they finally did in 11 those accounts would get banned. It would be counterproductive.

I'd be worried more about people being morons on it and arguing over it, honestly. But if there's a time limit in place it wouldn't be too bad.