Because Mercer is not good and doesn't really contribute in any meaningful way to the bottom 6 - he is bad at faceoffs, is bad defensively, isn't physical, and scores at a pretty pedestrian rate. He only had 4 more goals than Palat who everyone is ready to attach a sweetener to in order to ship out, and only 3 more goals than Cotter who is a 4th liner.
My genius idea, which might create some future problems but might help now, would be trying to convince Chicago to trade us Teuvo Teravainen + either Tyler Bertuzzi or Ilya Mikheyev for Mercer + Nemec + Palat + possibly picks. Teravainen would be an awesome 3C that could play up if Jack is injured, he has lots of juice offensively and could be paired with some more tougher/jam wingers like Meier or Noesen. Nemec is talented young D, which they desperately need, and are getting him for pennies on the dollar. I don't love Bertuzzi, but he's an improvement over the wingers we have now and I suspect if we give them Palat they'd want to move him back. He has 1 year more term than Palat but the cap number is better and he's a much more effective player. Mikheyev, don't know much about him, but he is only signed for one more year and makes sense as a veteran rental.
Money is mostly a wash.
Bratt-Hughes-Noesen
Meier-Hischier-Bertuzzi/Mikheyev
XXXX-Teravainen-Gritsyuk
Lazar-Glass-Cotter
Still need to dump Haula and get a middle-6 winger with some offensive skill (there are some decent vet wingers in this UFA class). But it's a start.