Re: Flexing (any benefits)
Ok, I think it's an isometric exercise, right? I remember reading that they can provide some strengthening but not much nor do they really build up alot of muscle. But isometric seems to be most valuable for people who need rehabilitation because of injury since no requirement of joint mobility to perform them and they can concentrate on the exact muscle that needs rehibilitation.
I guess the benefits would be very minimal if any for everyone else, though.