I agree that totally disallowing them would be shooting yourself in the foot, but there needs to be a balance between having them around and restricting their access somewhat. Not letting them go through 'endless loops' of things like sending a PM or contact email, or replying to a post makes sense and potentially helps save bandwith doesn't it? I'm not sure how that would be a bad thing?
Of course, I'm a newbie at this so maybe you know something I don't?