I'm a Niner fan but this isn't why I wouldn't start ARZ, I agree with a little bit of what Prane aka NinerHATER said but there is something he didn't putting into play is that ARZ and SF play each other like Cleveland and Pittsburgh play each other(not counting last game pit sucked). Those lowly Niners if you recall beat ARZ in their home and that it was a very close game.
Now depending on your league it could be different some leagues like NFL.com leagues yards and scores hurt you.
Yahoo leagues Yards don't hurt you only scores field goals really really hurt.
If your in NFL.com I would say not to start ARZ but if your in the yahoo leagues then you might want to start ARZ because it just might a low score game.
I was in the same predicament with the Giants and the Niners I chose the Giants D so I could switch out Percy Harving with Boldin cuz you never start a offensive player against your D.
It's ok for me I already clinched a playoff spot but yea it's more for a chance at the #1 seed.