Here's a little interesting history: back in 2003, Apple introduced 12" and 17" aluminum PowerBooks (remember the ads with Verne Troyer and Yao Ming?). The 15" model retained its titanium housing until later that year when it was updated to the aluminum design.
When I heard the rumors about the third notebook coming out at WWDC, I assumed something like that would happen again. Instead, as we know, Apple added its new 15" notebook alongside its current 15" notebook (the distinctions doesn't make sense: they're both new and they're both current). Anyhow, I think it makes less sense for Apple to have kept its current 15" MacBook Pro alongside the new model than it would make sense to axe the 13". While you can purchase a less expensive current than new 15" MBP, if you configure them with the same specs, the current is far more expensive than the new.
Back in 2003, it was clear that Apple would eventually get around to transitioning the 15" PowerBook G4. And now it seems clear that they will eventually transition to only "new" 15" MacBook Pros. I would also like to hope it's clear that they will transition the 13" MBP line, as well. Perhaps if you look spec for spec, you could say that the 13" MBP isn' t valuable compared to the MacBook Air, but as the hopeful next heir to the Retina display, it has great importance as a placeholder for what I hope will come. There are also some people like myself who prefer the glass display along with the MBP 13"'s display quality over the Air.
But again, I think the curiosity is that Apple kept the 15" MBP current. Apple often will force new technology onto the market even if it is prohibitively expensive for a while. Why they couldn't come up with a 13" "new" model simultaneously may be a matter of lack of resources to devote to two projects.
The other difference between now and 2003 is that then Apple expanded from one screen size to three, and now they are contracting from 3 to 2. I'm not sure if that made a difference in their decision to retain the current 15" MBP.
FYI: In the above any mention of current MBP refers to a non-Retina MBP and new MBP refers to an actual or potential future Retina MBP.