Unplayable Netflix Rentals

Hacking Netflix recently posted about unplayable Netflix rentals.  I've had this problem a few times recently as well.  A few have even been nicked up so much it looked like someone keyed the DVD.  Why does Netflix allow these to be in circulation still?  Not only that, but recently their service has slowed to a crawl.  Transactions which used to take two days now take four and five.  This is not acceptable.