On the other end, no one wants to deal with debt collectors, so it seems that they do provide a disincentive to defaulting. Still, the primary stick they have is the threat of a credit ding, which is there even without the debt collectors. And, with enough such dings, the borrower would no longer be able to obtain credit and perpetually abuse the system/drive debt prices down.
So, in this scenario, the market value for debt would seem more a function of the quality of the borrower in the first place. I generally don't think most people want to default and get dinged for it.
So, it seems that the biggest consequence is debt buyers going out of business. I don't know. I'm starting to have trouble seeing the real downside here!