The franchise tag doesnt seem like trade bait to me, but its not that bad of an idea. It gives them time to structure a long-term deal with Rudi while not having to fend off competitive offers. And since Rudi has said publicly he'd like to have a long-term deal with the Bengals, its not like he's angry being there, just probably annoyed like most players with the franchise tag because of the lack of job security it provides.
On a side note, whoever said they got nothing for Corey Dillon: look at who they drafted. Williams has become the strongest link in their secondary, and while the Pats certainly got a good end on that deal, so did the Bengals.