Arrington has never played in a 3-4, and I don't think he's the fastest learner in the world, if you know what I mean. If he was on the Cowboys, he'd be an occasional pass rusher who only came in on 3rd and long, or hail mary situations.
Also, he seemed to be dogging it in several games early in the season for the Giants. May he wasn't motivated, maybe he wasn't all the way back from an injury. Then he seemed to be coming around, and he got hurt again.
I don't have anything against the guy personally, but I just don't see much of a role for him here in Dallas.