In a reader mailbag on the Dallas Cowboys' team web site, the question came up of whether the team's planned switch to a 4-3 defense was a reaction to the read option offense of Washington with Robert Griffin III and the likelihood that Chip Kelly will run something similar in Philadelphia. The guys answered no, and I think they're right. Nick Eatman writes, "I think it comes down to the personnel and the Cowboys feel like their players are better fits for the 4-3 now."
Maybe. You could make the case that the switch will help the Cowboys run their defense through playmaking linebackers Sean Lee and Bruce Carter, and that DeMarcus Ware, Jason Hatcher and Jay Ratliff form the foundation of a very strong 4-3 defensive line. They would seem to need help at safety, and there is the question of whether the press cornerbacks they spent so much on last year in terms of money and draft picks are a fit with new coordinator Monte Kiffin. But the Cowboys' personnel seems better suited to switch from a 3-4 to a 4-3 than, say, the Eagles do to switch from a 4-3 to a 3-4.
I still think the economics of the thing have to be considered, though. The Cowboys are projected to be well over the salary cap right now, and re-signing Anthony Spencer seems unlikely. They could be switching to a 4-3 because that makes the most economic sense. Spencer would play defensive end in a 4-3, but if he doesn't come back, they could play Hatcher out there and find a defensive tackle to pair with Ratliff, or they could play Hatcher inside and find a 4-3 end. They have Lee and Carter coming back from injuries, and finding a strongside linebacker to team with them would be cheaper than trying to find a pass-rushing 3-4 outside linebacker to replace Spencer if they stayed in a 3-4. And with the depth at safety in this year's market, both in the draft and free agency, they should be able to find economically sensible solutions there as well.
Again, lots to a decision like this, but as usual, I think you have to consider money as one potential reason, and maybe the biggest of all.