See http://openborders.info/blog/immigration-trade-and-reciproci... . In particular:

"Now, the analogy between trade and migration is far from perfect. But I think that on the issue of reciprocity, the economists’ objection to mercantilist-style thinking in the trade context can be transferred to the migration context. In other words, unilateral open immigration generates net benefits, even if other countries don’t follow suit. Thinking of immigration law in reciprocity terms is fundamentally misguided."