Simple answer: a waste of the receiver's amps. What would be the advantage of a receiver over a pre/pro? If you have a use for the extra channels, or some feature not available in a pre/pro, maybe.

In my opinion, the only advantage of the receiver is easier hook-up, followed by the single-box advantage if space is at a premium. Remember, though, that amps generally don't have to be visible.

In my stack (which is six feet tall), the bottom two components are amps, but if I ever get additional pieces, I can always relegate the amp(s) to somewhere inside the closet. Flexibility can be useful.