I wonder what this does to the classic signability issues draftee. The guy who has commitments to college or the guy who wants top of the first round money? Can you afford to draft a guy who is a tough sign? Also, you lose some of the money from rounds 6-10 that you could spread to others, and you probably don't see a college senior drafted in the first 3 rounds to sign underslot to reallocate his money to others who fall (or to add to higher picks).
Edit - Elvir hit on some of these points while I was writing.