I think the whole Lefty - Righty thing gets thrown out the window if the player is too good to not play. Right now Koekkoek is knocking on the door and if he get's in later this season or makes the team next year, I don't see how Coop can deny him if he deserves to play. Coop has also broken this system before like he had to in the playoffs with a Coburn-Garrison pairing and he's trying to do now with Carle-Garrison.
Sustr I guess is honing everything. He's a master of no particular thing, who wants to keep his game simple. If in the end he can be more reliable, make good decisions on the ice, eat more minutes, and keep the puck out of the net, I'll take it. I see him trying to be more like a Mike Lundin.
NHL coaches like to use the 300 games mark as the point where a Dmen has enough experience to come into his own. Sustr could reach that in the year 2018, but who knows what the roster looks like by then.
Lefty-Righty is a big Yzerman thing though. It's why PK Subban nearly didn't make the Olympic squad, and why he was a healthy scratch most of the time. He was better than some of the lefties on that squad--not the righties.
Coburn and Garrison are both unique situations because they both trained to play the other side. Coburn, in particular, is a lefty that spent most of his career on the right. Those guys exist, but they're rare. (I think Nesterov might actually be one of those guys in the making, but that's up to management.)
There's a little bit of leeway because it's the third pairing, so it's a tad bit more relaxed and you'll see guys get into the lineup on their off-side for a game or two (Nesterov). But I think the days of giving full time spots to guys playing their off side are over.
With Sustr, yeah--he's honing nothing. He's trying to be an average defenseman who doesn't really do anything above average. Average skater, average shot, average everything. He's not going to wake up one day and have these basic skills somehow implanted in him. The only thing that's going to improve is his decision-making.