It depends.
For example, if a defenseman has the green light all the time to play offense, then scoring isn't actually any harder than it is for a forward. In fact it might be easier since the forwards are often occupying the defensemen (the best defensive players) on the other team and you are largely free to go up against forwards playing defense - all while playing minutes that are significantly higher than what a forward typically gets.
This is the case for Erik Karlsson - who won a Norris trophy, went to Pittsburgh, and somehow managed not to make the team any better (.521 winning percentage this year vs .555 last season). It's not a mystery, and it was entirely predictable. In fact it was predicted on this very forum by many people (virtually none of them Pens fans).
If Karlsson was the equivalent of a 131 point forward, as this poll result seems to imply, something tells me the Pens would have improved.
Maybe, just maybe, preventing goals against is a valuable attribute for a defenseman, and should perhaps garner a bit more focus than it currently does.