Schumacher has the right to do whatever he wants... and if Mercedes GP is crazy enough to sign him, that's their problem.
They already knew that a Nick/Nico combination would have been more ideal, but they made the conscious decision to take on Schumacher for purposes of buzz/publicity/marketing/whatever, and that's their boggle.
And Schumacher could have started better... Brawn GP had an odd couple in Button and Barrichelo last year, and that worked out well, with Barrichelo challenging for wins and points.
If the Merc GP were a better car, then we might complain about Schumacher. As it is... I don't see the pace.
I think McLaren is the big winner in the driver shuffle... they get Button, who's an arguably fantastic driver who still has many competitive years left in him.