I agree with Kenny. Right now Fitz does not have a serviceable backup. Thigpen was brought in because he knows the offense and if you remember him from the preseason last year, calling his play terrible is being generous.
Chan does not need the guy with the rocket arm and all of the other measurables to be successful in his offense - he needs someone that is smart and able to get rid of the ball quickly. Is it likely that Cousins becomes an elite QB, probably not, since you can say the same thing for just about any QB (other than Luck or RG3), but does he provide us a better option than Thigpen and a possible starter in a few years? I would say yes.