Lots of things to think about:
The Kings add $325K in cap hit to add a veteran to the blueline. Whether or not you like Phaneuf, it's still someone who will force Forbort, LaDue, and Folin to up their game since spots aren't guaranteed for all of them. Conversely, the Senators retain $2.5M in cap hit. Imagine having Gaborik for the next three years at his cap hit + $2.5 million.
Thompson is older and thus the Kings are trying to make a push for the playoffs. Maybe he'll push Mitchell out. I can only hope Carter comes back soon, regardless.
Shore was unappreciated here, but didn't fit in any long term plans.