This I agree with totally. I would actually feel pretty comfortable with a shut-down line of:
Powe-Halpern-Boyle
But the issue becomes, if this line is getting 10 minutes a game (not a stretch of the imagination), that decreases the ice time of the 3rd line which Callahan is on.
That is the issue with splitting up Gaborik, Richards, Nash and Callahan onto (3) lines while also wanting to have a "shut-down" line.
Interesting debate for sure.
Edit:
Also, something of note, when Boyle gets back in the line-up, if that is indeed the 4th line, the Rangers really only need a couple of other forwards to PK during the game. That means more rest for Nash, Stepan, Hagelin and Callahan when you have PK horses lie Halpern, Boyle and Powe on the bench. It also keeps everyone in the game and allows the coach to more evenly distribute ice time.