Some career percentages for elite offensive defensemen:
Bourque, MacInnis, Blake, Leetch, Zubov, Niedermayer, Pronger, and Lidstrom are in the 6-7% range.
Coffey, Orr, Potvin, Robinson and Housley are in the 9-10% range.
Those numbers provide a pretty solid story. Most of the "shooters" are in the 6-7 category; Orr, Potvin, and Coffey (though he really didn't shoot much after 1990-ish) are the exceptions. I would classify Lidstrom, Niedermayer, Leetch, Zubov, and Robinson as "passers", while identifying Bourque, MacInnis, Blake, Pronger, Coffey, Orr, and Potvin as "shooters." I really don't know what to call Phil Housley, since I've always thought of him as a passer even though he scored a ton of goals in the 1980's.
It's hard to judge, honestly. Some shooting percentages are obviously inflated by the high-scoring era and obviously deflated as those players aged. Take Housley for example; from 1982-1993 he was a consistent 10-11% shooter, but the moment the first lockout hit he went down to the 6-7% area. I guess 9% is fairly accurate for his career, however. It's a pretty damn interesting thing to look at, that much I'll tell you.
I still have trouble believing that Ray Bourque holds such a significant advantage in the shots on goal category. Considering he was often the best offensive threat in Boston, regardless of the fact he played defense, I guess it can't be that surprising, but 900 more shots than the next closest player? The fact that he and MacInnis had such deadly shots, and weren't afraid to use them, but still only scored on about 6.5%, really speaks to the difficulty of scoring goals as a defenseman.
Looking at overall rankings, it's almost shocking to see the rather pedestrian list of players at the top of the shooting percentage chart. I honestly expected Gretzky to be higher, but he's only 44th.