Advanced stats say that Murray actually benefited more playing with Wiz than the other way around, so I don't think he really has a "steadying" effect, and he didn't look very good in the games he played this season, albeit he's been injured. Tyutin I agree does have that steadying effect, though.
One thing I reallly like about Johnson, is that he seems to be able to get the team going with some really big hits, and he can really bring guys down. I've seen it somewhat in Cotton and Wiz too. When guys start throwing hits, I think that's contagious.
I think the comparison between the defensemen and the forwards is somewhat of a false dichotomy, though. The defense doesn't rotate like the offense does, and the difference in playing time between the last pairing and the first pairing is often negligible. Often times the top forwards are playing 20 minutes a night, whereas guys like Boll played barely over three minutes in Dallas. That's a difference of 17 minutes.
The difference in playing time between the top pairing and the lowest pairing is closer to the difference between the first line and the second line offensively. Not to mention their job is completely different, so I think that comparison is rather problematic to begin with.
I think that people almost automatically value players (defense included) based on their role. People aren't rating Prout low, because he doesn't score like Wisniewski. Also, people rate Wisniewski high because of his value to our offense and power play (although his defense has improved immensely). No one expects Wiz to be a defensive juggernaut, but he fills a huge need for the team, he has his role and he excels at it, thus his rating in the top 3-4 defensive players. People rate Tyutin high, despite him not standing out really in any category, but just because he's consistant. It's the value of the player within the parameters of their roles that the ratings are based on, I think. Which, is probably what you were saying, but I think became somewhat muddled when you brought in a defense/forward comparison.
I think that when evaluating players, people automatically look to how they fit individually into a whole. If their individual skills are synergistic then their individual short-comings will often be overshadowed, and filled in, by the core competences of other players or their respective partners.