As usual, a number of excellent comments.
Several express concern that an Indian "brain drain" will hurt India by depleting its supply of high-IQ individuals. This is unlikely. As I said in my post, the "drain" is self-correcting because a reduction in the supply of (say) software engineers in India will result in higher wages for those workers there, which will not only slow the drain but also increase the supply by improving the job prospects. Since India has a population in excess of 1 billion, the number of high-IQ individuals is undoubtedly so great that a limited brain drain will not significantly weaken the nation's long-term prospects.
Several comments discuss my puzzlement concerning the political opposition to larger quotas for skilled immigrants. I can understand why the skilled American workers with whom those immigrants would be competing might favor keeping the quotas low, but my impression is that for the most part they do not, perhaps because in knowledge industries like software skilled workers add more value than they capture in their wages, creating a virtuous cycle that benefits the entire workforce in the industry. (This would be consistent with opposition to expanding quotas for women and minorities--such quotas favor the less skilled and so do not produce value that benefits competing workers.) The main opposition to relaxing the quotas seems to come from unions; maybe they fear that any relaxation would spread to less skilled workers. The universities, moreover, have an additional stake in limiting the quotas besides the one I mentioned in my post; as one commenter points out, foreign students, forbidden by their student visas to work at regular jobs, provide valuable research assistance to university faculty.
A comment about the cultural benefits conferred by immigrants suggests a partial answer to the security concerns that I expressed in my post. Our intelligence system is in desperate need of more people fluent in Asian (including Middle Eastern) languages and intimately familiar with the cultures of those regions, and the need cannot be met by training Americans.
Several comments emphasize quite properly the defective structure and administration of the quotas. One comment points out that as the temporary skilled-worker visas (H1B) expire, the holders often join the queue for permanent-residence visas, and so the queue lengthens--it is now several years for India and China. (The quotas are on a country basis--another mistake.) Visas are granted in the order in which they are applied for, and thus to the people who have been in the queue the longest. They tend to be the less qualified workers; the more qualified will have had better opportunities in their home country and the longer the queue, the more incentive they will have to exploit those opportunities rather than wait in immigration limbo. In addition, we lose some of the best immigration prospects by delay in the processing of visa applications, which is due to a shortage of visa personnel against a background of increased scrutiny of those applications for security reasons. The relatively low costs of expanding the number of such personnel would probably generate more than offsetting benefits in a higher quality and quantity of highly skilled immigrants.