Thinking about free agency in the sixth round is pretty stupid, in my opinion. You should still be focusing on your board and drafting your highest rated players who might have slipped, not rounding out next year's practice squad. This is the same way they lost out on Thomas Rawls. They thought they could sign him as an UDFA, so they drafted a crappy tackle instead, even though Rawls was clearly the better player.
I think the Cowboys' theory makes more sense in the 7th round, since fewer players will be drafted after we draft and the talent level is close to being depleted anyway. It is a calculated risk taking a player you like less, because you believe a player you like more will sign with you as a free agent.
This isn't unheard of, since we did it with Witten, and let him slide a round and we still got him. That worked out pretty well, though the risk in that case probably wasn't a good risk.
We did the same thing with Demarcus Ware. Few people remember that Spears was drafted behind Ware, but we actually wanted Spears MORE, but we thought Spears would last until our later first round pick, but they were confident that Ware would not. There is risk in all of this drafting stuff.
One more ...... we traded down and left meat on the take with Steven Jackson to take Julius Jones to get a second first round pick in the following draft. While that was a poor decision in that draft, we used the pick the next year on Spears. If we did not have that pick, it is a distinct possibility that we would have taken Spears first with out #1 pick, and lost out on Ware altogether.