Well, we got Willis for a 7th round pick... In a vacuum (i.e. just looking at Willis' value), he is a FA next year and probably not a future starter - certainly not for the Bengals.
But Willis on the current Packer's team is pretty valuable for insurance purposes (ironically the very reason the Bengals wanted to trade for him). We have pretty high hopes for a SB, so he needs to stay. But if we were a 500 team, would we scoff at trading a backup QB that we probably won't retain next year for a 6th?