The guy's got a career +/- of 300. Yeah, that's terrible defensively! What's more effective defense? Grinding in the corner, fighting with another player for the puck while 2 more of his teammates skate in to fight it out for the puck, or skating away with the puck to score? Yeah, checking, grinding, positioning are all important aspects of defense, but they aren't the only aspects of defense. Possessing the puck and being on offense means that the other team isn't possessing the puck and being on offense.
Another thing to consider - not only was Coffey capable of that, he was capable of containing the transition to one player. Every pass you make is a pass that can be intercepted. Coffey was capable of transitioning from defense to offense without making a single pass. Is that what he did all the time? No, but it meant that the transition was more difficult to contain because Paul had that extra option.
Paul the best defenseman ever, or more deserving than one player or another for the Norris trophy at any given time? That's subjective, and I'm not going to throw in my opinion - but to say Coffey played terrible defense, I strongly disagree.
One way to look at it - defense as a single dimension is useless. The point to defense is complementing offense. Paul wasn't that one dimensional defense, he was both. The goals for as opposed to goals against is evidence of that.