I think there are a few agents who ask for exclusives because they don't want to invest the time to read something, only to be 3/4 of the way through and be told the author has accepted representation elsewhere.

One well known agent worded it like this - "I'm at a stage in my career where I don't need to be competing with other agents."

I think the agents who ask for exclusives are in the minority. It will be interesting, as time goes by, if that changes?