Jackman is not the worst defensive player on this team, he is more likely the best or 2nd to Polak depending on what you consider the most important variables of being a defenseman. I can show you this if you like, but since you (general "you", nobody specifically) have seen the likes of Rick Nash and other superstars frequently make Jackman look bad then he must be worthless, right?
Two things make me view Jackman as the best choice to trade away....
#1) He has not had his head in the game what so ever. Dude is missing it between the ears right now and I saw clear signs of it last last year was well. IMO he doesn't 'love' hockey anymore and that is not something likely to change.
#2) Contract situation - he still has two years left on his deal. If you get out from under his contract, it is reasonable to think you could pick up 2 mill of Brewers contract and unload him in the offseason and now you cleared two wasted spots and 6.5 mill to tack on top of Paulie's 6 mill, Walt’s, 2 mill, and Mason's 3 mill.
Resign Carlo, and elevate a kid and you'll have as good of a D (if not better) and 17.5 mill to get a Goalie and some scoring .
Oh I'm not against trading him, by all means get some value for him. He is probably a little overpaid, but he does his job as a defensive defenseman just fine. He hits, he blocks shots, he roughs it up. He has the highest +/- of all defenseman on the team.
He and Weaver put up almost 4 minutes of SH TOI per game each, good for tops in the league. What else is near the top in the league? The Blues PK.
He is nowhere near perfect. His TA/GA ratio is horrible, twice as many give-aways as take-aways. But to act like teams should consider him a low-value player is insane. He is just the player I'd want on my 2nd or 3rd (if I had a deep team) defensive pairing and on the penalty kill. To quote Illinois' recent governor "That's a f***in valuable thing!"