I'd think it would be easier to lose track of the front blade behind a GR sight, but if people are shooting faster with them then that's fine. Then again, you can always shoot fast enough to miss, so it seems like a draw anyways :P .
And if you're looking at IDPA, they aren't allowed at all.
"An armed society is a polite society." Robert A. Heinlein
"After a shooting spree, they always want to take the guns away from the people who didn't do it. I sure as hell wouldn't want to live in a society where the only people allowed guns are the police and the military."
William S. Burroughs