I watched a lot of Callahan. I wouldnt say hes a defensive specialist. He's not stealing passing, picking pockets, etc. He's a hustler and annoying to play against. He's always working his ass off, rarely gives up, and he's always right there. When you are in his zone, he makes you go through him.
But, I do notice that he doesnt back check after he loses the puck in the offensive zone. No one believes this, but I notice this all the time. Probably why the +/- isnt always respectable.
You're not wrong, but you don't have the full story, I think.
When he's on the rush and loses the puck, he doesn't dart back on the forecheck, he almost always mans up with whichever near defenseman is likely to join the rush. The result is some plays broken up, but it doesn't always work. That's 1 part of his +/-.
The other is that he's often thrown out against the best competition. They don't usually want to put Gaborik or Hagelin out of the ice against Sid, Geno, OV, Kovy, or those guys, but guys like Callahan and Stepan, and when they were with us, Anisimov and Dubinsky, play very good defense while still maintaining a legit scoring threat. The Rangers 2nd line almost always ends up paired against the opponents 1st.