Firstly this is just wrong. Investment banks (or those banks formerly known as investment banks), are defaulting because they can't borrow. They can't borrow because nobody knows the value of their balance sheet.
Secondly, calling mortgage backed securities "practically worthless" is a strech. How many people that you know are currently in forclusure? Lets assume that 10% of people with mortgages are in or will be in forclosure (this number is ridiculously high). We'll also assume that a house in forclosure is absolutly worthless (not true). What is the value of the mortagage backed securities backing these houses? 90%.
The problem is nobody knows who will be forclosed upon. It could be that in a given bond 90% of the houses in it are being forclosed upon. But you can't really know this. So nodoby will buy them. The number I heard, and I could be totally wrong, is that mortgage backed securities right now are selling for 22 cents on the dollar. Do you reall believe the value of the US housing market is down 78% in the last 2 years?