It's unknown exactly what the terms of the trade will be. Scuderi's contract runs through 2016-17 at a $2.25 million cap hit, which would be very problematic for the Kings to take on. Perhaps the Blackhawks will eat a chunk of that (the Penguins already are), or perhaps there is a Vinny Lecavalier-esque agreement that Scuderi will retire. (Jon Rosen implies it will be the former.)
In any case, while Ehrhoff is not much of a loss at this point (he hadn't been anything special this year, and LA was clearly done with him), Scuderi doesn't have much to offer to the Kings. He's seriously declined from his days as an acceptable defensive defenseman and is now a big on-ice liability--hence the Penguins and Blackhawks shipping him off for nothing this year.
Hopefully, LA won't give Scuderi any playing time and simply expects him to hang out in the locker room and be the seventh defenseman for the playoff run. I would argue the Kings don't need Scuderi's veteran presence to win the Stanley Cup, but I guess it can't hurt (so long as Scuderi never takes the ice).
If he does take the ice, well, that would be bad. Don't do it Kings!
UPDATE (2:10 PM): The Kings and Blackhawks have officially finalized the deal. In a corresponding move, the Kings assigned Kevin Gravel to the AHL.
Scuderi to Kings for Ehrhoff is official. Gravel assigned to Ontario of AHL— Helene Elliott (@helenenothelen) February 26, 2016