Alan, yea that makes sense.  It also makes it all the more perverse.  They avoid the anti-predatory lending laws by selling as a "for sale by owner" then immediately dump the loans into the secondary market.  Brilliant!  I should have been a banker!  (not really, I wouldn't be able to sleep at night if I made a living doing crap like that.)
Oh and they don't securitize these packaged loans every few months.  There is a market out there that satsifies this demand every single day.  This is how Fannie Mae/Mac are now on the hook for something like over 40% of the total market and over 60% of the sub prime market.  I would love to get updates on those numbers though as they are about 6 mos old and from memory.