Well, I am all about having a group of Dmen who can actually play D, but there is something to be said for an offensively inclined Dman. I think you only need one or two on your group. They can come in all shapes and sizes, but the one that is a a bitch to defend as an opponent is the guy who can skate like the wind, and has puck skills to go along with that. Those types get more room to wind up, and end up hitting the opposing Blue line at full speed, while the opposing Dmen are already backed up and are either stationary, or have lost their gaps, making it really hard to defend. Also, most teams defensive schemes are about accounting for forwards. Throwing in a Dman on the attack can really confuse and overwhelm the oppo.
We saw what Dumba could do for a little while. Guys like Makar are the ultimate...well, Orr was the ultimate... the downside of these types is that defending can often be of secondary importance to them. If they are prone to giveaways it can be disasterous.
There are other types like Carlson, Josi and Burns who really on smarts and the big shot, rather than skating.
Bottom line is that having an offensive Dman or two in your lineup adds a new wrinkle to your offense, and yet another tool to break down a defense.