+/- is only good in comparing players on the same team. Even then it has limited value when players play entirely different roles. But when on the same team, one guy plays a predominantly defensive role with a high QOC and low offensive zone starts and the other plays a predominantly offensive role with a lower QOC and a high ratio of offensive zone starts, and the guy in the bad situation looks much better, it says something.
Advanced stats also bear this out. When you look at Corsi adjusted for QOC, the rankings on the team are Chabot, Ceci, Phaneuf, Wideman, Karlsson, Claesson, Harper, Borowicki. And in corsi adjusted for quality of teammates he is only second to Phaneuf and only one of two D men above 50%. And he gets noticeably less offensive zone starts than Phaneuf or Chabot.
Those advanced stats indicate that when factors such as linemates and competition and zone starts are accounted for, he has the best advanced stats on the team, and that bears out in the +/-.
I'm certainly not saying he is the best D man on the team, nor even a great one, but what he does is eat up the toughest minutes on the team and plays others to a stand stand still in one of the hardest to fill positions in the league. You need guys like that to win games.