1. Brandon Carr's contract.
According to overthecap.com, Carr will count $12.2M against the cap this year, $12.7M against the cap in 2015, and $13.8M against the cap in 2016. Carr is solid, but not spectacular. However, he's getting paid like a top 5 league corner (the franchise number for CB's this year was $11.8M). While he's not going anywhere this year (and probably not next year, either), cutting Carr after the 2015 season would result in a cap savings of over $6.3M. Couple that with the fact that he'll be 30 years old by then, and it seems likely that Dallas will be without their best corner within two years.
2. The uncertainty that is Morris Claiborne.
I was a fan of the trade up to get Claiborne. I felt like he was an impact player at a position of premium importance in the NFL. He may still prove to be just that, but to this point has shown very little that he can be counted on to perform as an average starting NFL corner, much less a top-tier, lockdown defender. This upcoming season is very, very important for Claiborne. If he again struggles to adapt to the NFL game, Dallas has to find his replacement. Some players get it together and go on to have great careers after struggling for their first three or four seasons, but only very few (and even less with their original team).
3. Depth at DB is important, too.
Having a ferocious rotation on your defensive line can (and generally does) lead to winning football games. Getting consistent pressure from your d-line is extremely important in today's NFL. But so is having multiple good DB's, and for much the same reason (teams are dropping back to pass more than ever). Having a stable of really talented corners does not sound like a bad idea to me, at all. Teams are putting three and four wide receivers on the field more than ever, which means your third and fourth DB's need to be talented players capable of holding their own.
4. At least one of the top 3 CB's is likely to be available at 16.
In rabble's article, none of the CB's get picked in the first 15 picks, which means Dallas gets first crack at a very talented trio of corners with their first rounder: Justin Gilbert, Darqueze Dennard, and Bradley Roby. Gilbert and Dennard, especially, are tantalizing possibilities. Gilbert is a world-class athlete with a great size/speed combo that had great production in college and is the best returner in this draft, to boot; I feel like he's criminally underrated in this draft. Dennard fits Dallas' current defensive scheme to a 't' and was a playmaker on one of the nation's best defenses at Michigan State. Roby had a down year in Columbus but was a big time player before this season and re-opened some eyes by showcasing elite athleticism during the pre-draft circuits.
5. Drafting value at CB instead of reaching for DL is a long-term move
Everyone wants to believe (and many already do) that Dallas' brain trust has changed its ways. No longer is Dallas the "splash play" franchise like the Redskins; Stephen Jones, Jason Garrett, and Will McClay are largely in charge of the direction of the franchise and are committed to making smart, cap-friendly decisions that benefit the long-term health of the Cowboys. This may be wishful thinking, however. All it takes is one big signing (Chris Johnson?) to kill dreams and show that Jerry is still in charge. In my opinion, drafting a Justin Gilbert or Darqueze Dennard at 16 because you have them higher on your board and then addressing the defensive line in the second round is a long-term move.
In all likelihood, Dallas will need at least one new starting cornerback within two years. Where do you want him to come from? Another expensive free agent like Carr? Or would you rather have someone already in place that was a great prospect coming out of college and is on an extremely cap-friendly rookie contract? Personally, I'd be happy with several different positions/prospects that look like a possibility in the first round. A pairing of Gilbert in the first and someone like Scott Crichton in the second, though? I'd feel like Dallas knocked it out of the park.