Shot power isn't everything. It's knowing when to throw it on net - when it will be most likely to be tipped, go off something, get a good rebound, etc.
Bieksa is a great example of this. His shot is pretty weak. But he knows when to use it, and usually gets it exactly where it needs to be. As such, he creates a lot of offense - 2nd most ES points in the league (by a D) last year.
Exactly, for defensemen, you don't have to be a sniper to create offense. It's more about timing, placement, jumping up in the play, etc. You could have the hardest slapshot in the league, but it's still an easy shot for the goalie to stop unless you let if off at the right point.