To begin with, the Caps already have Mike Green and Tom Poti locked up for the next couple of years or more, so add Erskine and that's three NHL rearguards (insert joke here) under contract for the next two years.
Beyond that trio, the Caps obviously have high hopes for Karl Alzner, and have shown differing degrees of faith in 2004 draftees Jeff Schultz and Sami Lepisto. You'd expect all three to at least get a fair chance at a spot within the next two years (Schultz obviously already has one), and Tyler Sloan is currently getting an audition in The Show. Other prospects (John Carlson and Josh Godfrey, for example) are a little further off, time-wise.
Which brings us to Shaone Morrisonn and Milan Jurcina. It's pretty clear that this is a fish-or-cut-bait season for Juice, but I wouldn't be so sure that Mo has a spot sewn up yet. He's more than twice as expensive as Jurcina, and, frankly, still needs to prove that he's worth it. Yes, he's tough. Yes, he plays big minutes against top competition. And yes, he covers Mike Green's backside. But he's also just a couple of months removed from beating the team in arbitration (after a contentious hearing) and if the Caps had full confidence in him (there are indications that GMGM might not even consider him a top four guy), they'd have signed him to a long-term deal by now, no? I'm not saying they won't, I'm just saying they haven't.
The point is that the Caps look to carry seven defensemen for the foreseeable future, and with Erskine signing an extension, the number of spots up for grabs (barring a trade or a relatively cheap buyout) is reduced by one. You do the math and tell me who the odd man out is.