Jeff,
Those who propose filtering a la Verio / Sprint(passim) suggest that your incentive to renumber is that certain other (not in the line of transit) networks will not accept these prefixes (or apply more stringent dampening on them), and hence give you inferior routing either permanently (filtering) or temporarilly (dampening), assuming you have a covering netblock.
But that's not an incentive to renumber at all, because I can't go to ARIN and say, "I want to renumber out of these disparate blocks and get one big one that is more globally routable." So renumbering out of the block that I'm thinking of (204.252.74/24, FWIW) still doesn't do me any good.
Well, I'm more familiar with RIPE than ARIN, but if you are saying 'If I were to apply for a /x anew, ARIN would give it to me, but as I already have a /a, a /b, a /c, ARIN won't let me return them, and renumber into a /x' I'd suggest that policy needs looking at, for exactly the reasons you suggest. I /do/ know of instances where companies have (say) an old /16, severely underutilized, and want to get more space for some reason, offer to return their old space, but insist on getting at least a /19 (or similar) on the grounds of routability, even though if they made the application afresh they'd get at most (say) a /22. Hard one to call that. -- Alex Bligh Personal Capacity