I think this would only help the Big Market Teams. Small Market Teams often shy away from or are less inclined to take a chance on signing a a draftee to a huge signing bonus because they don't have the money or would rather spend it on something that is not so risky.
Sort of Reminds me of the Reds drafting Jeremy Sowers. They knew he wasn't going to sign, but that meant they could pocket the $2-3 million signing bonus money. Without a salary cap, the trading of draft picks wouldn't work in our advantage.