Higher rates for lenders. 
Did Prosper just do something that would give lenders higher rates of return?!
I didn't read where they are actually going to perform collection activity. So why would a borrower be any more inclined to pay a note today? No matter what the stated APR might be.
I think the theory is that this will keep newbie (and overly optimistic) lenders from bidding down rates too far. All else being equal, higher rates (with constant defaults) will result in higher returns for lenders. Of course, the "constant defaults" part may well not be true, if the higher rates instead lead to a lower quality pool of borrowers who find it advantageous to use Prosper.