In essence, most of these folks are saying, "It's hard to enforce the law if you dont' know who's violating it."
If you sit down and think about that for a minute, the corollary to that is, "It's hard to enforce the law on people who aren't creating any problems --- no neighbors are complaining about them."
And then, "It's hard to enforce the law on people whose only violation is on paper, and they're not causing any problems in the community, because if they were, we would know who they are."
Ultimately what it comes down to for Arlington and so many municipalities, is the fixation that cities have on trying to fix problems that don't exist, except in some technical sense on paper. Why are you so concerned about what people are doing when there are no complaints about them? If you're fixated on strict adherence to the law, and making sure no one violates the jot or tittle of any law, well there are about 12 million illegal immigrants in the nation...get cracking.