Basically, it's like this:
Parrish has character and a lot of on the field saavy. He's lost a step or two and while he has a nose for the ball, he was never a top notch cover guy... so losing a step will hurt. His ball-hawking skills and instincts for the position will make up for some of the lost athleticism, but not all.
Everything being equal, I'm sure that San Fran would've kept him on the roster. However, with this season basically done with for them and them being a team that is working on building with youth for the future, keeping the younger developmental guys over an older, character guy on the downside of his career is a no-brainer. He might add to the locker room and he might make a few more plays on experience and saavy, but the developmental guys might turn in to a real player in a year or two and by that point, Parrish would be gone. It makes perfect sense for the niners. Throw in the fact that he makes a lot of money and the young guys don't and frankly, the niners would have to be stupid to keep him in their current situation.
For Dallas, on the other hand, they're making a push now and Parrish's locker room presence and on the field experience might very well be the difference in a game this season. In a playoff game this season. One veteran decision by Parrish could be the difference between Dallas going home or Dallas going on in the playoffs. Throw in the fact that our FS play has been somewhat less than stellar, and it is easily a good move for Dallas.
It was also a good move for San Fran.
I understand why the niners would want to cut Parrish and keep the young guys. I also understand why Dallas would want to add him to their roster to see if he can make a difference in their playoff push.
Works for Dallas, San Fran and Parrish.