I look at it this way. Justin Smith brings few, if any, positives into a sea of negatives on this defense. He has a motor that will never question his effort. BUT. A six-year veteran that was expected to be a play-maker/sack-guy, should have more than 41.5 career sacks. He's far from competing with the top of the class at his position. The Bengals shouldn't sign or tag Justin Smith because the expected demands wouldn't apply to the overall scheme to retool the defense -- if indeed that's what they plan on doing.
When Smith is on his game, he's tough for offensive linemen to counter. However, his consistency is anything but. He had six games with two tackles or less. Take away the three-sack performance against Kansas City and he has four in the remaining 15 games. He went on an eight game stretch without a sack (career worst). When the Bengals hit their season high four-game winning streak, Smith had a total of seven tackles and no sacks. In other words, the Bengals defense was good during that stretch when Smith did nothing. That's not impressive for a guy that had arguably his best season. Heck, you could even make the claim that Smith's numbers are inflated because of a few big games among a bunch of games where you wonder if he's even playing.
Consecutive games without a sack. Note how the past two seasons where he increases his long stretch of games without a sack.
The Bengals defense was ranked 17th during Smith's rookie season (2001). Since then, they've dropped to 28th, 19th, 28th and 30th. This season was the first, during Smith's tenure, that the Bengals scoring defense ranked below the twenties. I'm not correlating this to specifically point out Smith is the problem. Rather Smith doesn't stand out to make this defense better. He's a part of the status quo.
Now, whether or not the Bengals should sign Smith is primarily based on his demands. If the money reaches the Robert Geathers zone, then I don't see the logic in signing Smith. This defense needs to retool and turn their personnel around. Signing Smith to a large contract will prohibit that. If the team tags Smith as a franchise player, then that would be a waste of $8.6 million that could be reappropriated to, perhaps, a play-maker -- rather than sticking with the status quo.
If the Bengals want to stay with the status quo -- crap, undependable defenses that give up 42 points in a half to a great team you should have beaten -- then go ahead and sign him. While you're at it, extend John Thornton and re-sign Tory James. Either way, with this defense, more 8-8 seasons aren't out of the realm of possibilities.