For the open-borders crowd, immigration is a patent medicine, able to cure whatever ails you.
The latest pitch is that requiring immigrants to live in Detroit is the solution to that city's many ills -- see here and here, for instance. The second link, from National Journal, is aptly titled "A Modest Proposal", because it's no more real than Jonathan Swift's 18th century essay by the same name calling for the Irish to escape poverty by selling their children as food to the rich. The problem is that the Detroit suggestions do not seem to be intended as satire.
And yet, what they're suggesting is serfdom - legally tying people to a particular place on pain of expulsion (not that expulsion would ever happen anyway, since people would just leave Detroit as soon as they could and become illegals elsewhere). When Agriculture Secretary Tom Vilsack was governor of Iowa he suggested a similar effort to make up for all the Americans leaving Iowa.
All proposals like this have one thing in common -- they objectify immigrants, imagining them to be something other than human beings, nothing but variables in an economist's fanciful equation. It's the geographic version of the "Jobs Americans Won't Do" fallacy - "Places Americans Won't Live", if you will.
In fact, immigrants have the same motivations and goals as the rest of us. And if Detroit is so badly governed, or Iowa is so remote and frigid, that Americans don't want to live there, why would immigrants? The same factors that lead Americans to move elsewhere will lead immigrants to do the same. Immigration-as-magic-bullet ideas are just cop-outs, a means to avoid facing the underlying problems of such places, either because there's no way to change those problems (bad weather) or because it's just easier to weave fairy tales than to deal with crooked politicians, grasping unions, and a degraded electorate.
What it boils down to is this -- immigrants go where the jobs are, not the other way around. Detroit can only be fixed by Detroiters (if at all), not by the intervention of magical foreigners.