I guess the reasons are better shifting and lighter weight. I'm not sure if those are compelling reasons, but that's a decision everybody has to make for himself.
I know my LBS had a customer who was explained that the Scott CR1 had a non replaceable RD hanger, but that was the one he wanted. So he wrecked it and showed up at the LBS screaming every single day for an entire week and then got himself a lawyer to sue Scott. I believe the lawyer was able to talk him out of it, but I'm not sure.
I'd be curious to know what would have happened with a fixed cf rd hanger. Either it would have snapped or the rd would have gone into crash mode or that would have been wrecked. Of course it could have been a combo of the above.
I do know that if the hanger was toast I'd have a very nice track bike on the way to me.
I do sorta like a2j's approach with a fixed RD hanger that can be replaced in the factory, but I suppose that only works for frames where you can pretty much be sure the owner has a couple of other bikes to ride in the meantime.
"Nothing compares to the simple pleasures of a bike ride," said John F. Kennedy, a man who had the pleasure of Marilyn Monroe.