I'm not sure this trade really benefits you. It's seems pretty even to me. I think you'll lose a little something at WR, as Philly seems to be a little more high powered that SD right now. And Gates is picking up some more catches for SD. Although V. Jackson is still a stud and has really only had one less than stellar game. I don't know when D. Williams will get back in to form, or if he will continue to lose touches to Stewart, but it seems Jacobs is on a better team with opportunities for more touches, even with Bradshaw in there. But I don't think the gains you'd get with Jacobs warrants this trade.
D. Williams AND Jacobs have underachieved this year, but I do think Jacobs is more valuable. D Jackson and V.Jackson have both OVERachived this year, but I think V.Jackson is more consistent and more of a go-to guy than Desean at this point. So as someone else said, you improve at both positions (although perhaps slightly), so it seems like a good move to make. Go for it.