Well, because there should be, and here is why. Let's look at the 2008-2009 season. In it, we will assume that the top-six defenders were as follows:
That group of six defenders played together in 25 games last year. Playing together in fewer than one in three games is indicative of the injury/illness issues with which the defense had to contend last year. Whether that is extraordinary compared to other teams, we don't know, but it isn't relevant to the point we are about to make.
In those 25 games, the Caps compiled a record of 15-6-4. That would be a 113 point pace over 82 games. In the other 57 games the Caps were 35-18-4... a 106 point pace. Was an intact defense that much superior to what at times was a make-shift defense? One could say that it was a case of guys like Karl Alzner, Tyler Sloan, Bryan Helmer, and Sean Collins stepping up when needed, but those guys were in Hershey for a reason, just as the six we mentioned were the top six defensemen in games played. Presumably, the latter group is better.
But were they that much better? Even if we introduce the factor of a Brian Pothier return, it didn't seem to matter a lot. There were an additional eight games in which Mike Green and Pothier played, with a mix of defensemen filling the other four slots. In those games, the Caps were 5-2-1, a 113-point pace (although the small number of games doesn't lend itself to much in the way of conclusions).
If you take the combined "top-six" and "Return of Pothier" games, that's a collection of 33 games in which the Caps went 20-8-5, a 112-point pace. It didn't seem to matter in any demonstrable sense which six players -- as a group -- were manning the blue line (conversely, maybe it's just a case of the forwards being so good as to overshadow various mixes of defensemen).
We could be accused of splitting hairs when no matter which of these groups of six we're looking at, we're looking at a record that reflects a 100+ point pace. And given how bad the Caps defense was a short few years ago, we might be seen as splitting the split hairs. But we would think that a "top-six" group would matter more. And if the Caps are going to seriously contend for a Stanley Cup this year, we suspect they will have to matter more.