All parties involved have already caused crippling damage to the economy.
They sure have, but NOTHING like what I understand would happen should they be allowed to collapse completely.
I don't fully comprehend the monetary system in the US (I've got a lot more of it figured out than the common Joe, but not as much as I'd like to...), but I think I understand the general cycle.
Without Fannie and Freddy around to gobble up all those toxic bad loans, that loss is going to come straight out of the real economy. There isn't enough money in the system for that to possibly have a good outcome. We're talking 50% of all home loans here. That is an astounding amount of money! To have that loss hit the real economy would be pure chaos. Major bank run and massive liquidity crisis ensues. Cue a likely complete economic collapse, and possibilities as wild as complete US bankruptcy or governmental/systemic upheaval.
With Fannie and Freddy under the Fed, it'll almost work out like all that money never existed. You have the Fed pump a bunch of funny money into the system to fill that void of toxic bad loans, the money subsequently disappears (those loans and monetary instruments are worthless...), and the rest of the system ends up fairly untouched. Hopefully.
Sure, there will be some (probably significant) inflation damage... Sure, the housing market "correction" will hurt... Sure, we'll piss off some (most? all?) foreign nations... But I'd still rather see that to avoid the chance of a complete collapse.