That was the argument for Fanny and Freddie. Didn't seem to work, and I bet it won't this time. WAMU is next.
To make the pro-bailout argument more clear, here is one reputable economist predicting that if we had let the FMs default, the dollar would drop about 30% and unemployment would double, in about a week.
http://www.marginalrevolution.com/marginalrevolution/2008/09...
Obviously, these bail outs have their costs, such as the value of money lent as well as increased moral hazard, but it's worth considering whether this is worth the potential cost of a market collapse to the whole economy.