I think this move probably saved the FDIC money. If you had a massive bank run(1), it's probably cheaper to pay back 250k+ depositors of one bank than for the FDIC to pay out < 250 on 40. Not to mention unloading enormous amounts of MBS's could trigger further insolvencies.

1. which seemed pretty likely, I know people who over the weekend were planning to pull all of their money out of a regional bank and put it in one of the 4 super banks Monday morning)