Why do old GM shares still trade?

One of the many things that undermine confidence in stocks is that the shares of bankrupt companies continue to trade. Those shares are completely worthless and yet those who are so inclined can trade them, and sometimes make enormous amounts of money in the process.

This comes to mind in considering the behavior of the common shares of the old General Motors (MTLQQ) -- the one that includes businesses that GM is trying to sell, such as Pontiac, Saab and Saturn. The shares should have no value because there is no cash expected to remain for the common shareholders after the sales.

And yet those shares traded at 52 cents, up one cent on volume of 32.8 million shares on July 21.