After reading Jason Whitlock's
article on BSPN, I'm thinking Mike might be making a last gasp effort to vindicate himself by starting Cousins. I'll explain in a minute.
Think about it. Sure Cousins is your #2 guy, and you probably want to showcase him, but why not just start Grossman instead of taking the chance on Cousins getting injured? It's because you feel you legitimately have a GOOD chance to beat Atlanta and Dallas with Cousins at the helm. Sitting Griffin does two things. It humbles him into working even harder to mature as a pocket passer during the offseason, a la Cam Newton, and it ensures he's healthy in order to mature.
Showcasing Cousins isn't the ONLY reason he's starting these last three games. Within the framework of the team, the FO and the league, Mike would show he was right to sit Griffin should Cousins light it up and win at least two out of three. Will it save his job? Probably not since he's made too many mistakes, IMO, to justify being kept around for Year 5 but he will achieve a sense of vindication to himself, the team and, perhaps, the league if the team can rally around Cousins and Captain Kirk is able to do what RG3 couldn't, i.e., complete passes, avoid sacks and extend drives with his arm.
However, should Cousins produce the exact same results as RG3 then all bets are off. There will be no vindication for the Shanahans and Mike will get his deserved canning. He will, however, have done a huge favor for the next coach by allowing him to develop a HEALTHY Robert Griffin III. Any way you slice it, sitting him is the right thing to do.