Speaking english is a certainly a plus, but it seems outweighed by all the other benefits to the advisors that a foreign student brings.
(Full disclosure: I'm an American who's going to apply next year to PhD programs.)
But, more likely to be pressured by their family into a "prestigious" career that they aren't really interested in, more likely to have the additional pressure of supporting a wife and be thinking about kids, more likely to be unable to learn how to write research papers well in English (this is surprisingly hard)...
Also, I've definitely seen people from abroad use American grad school as a temporary stepping stone to get a job at a big company (like Microsoft) that will deal with the visa stuff. Not saying they were planning it all along, but it happens.
Of all the above, I think the "being able to write polished, professional, technical research papers" is by far the biggest risk factor that neutralizes the benefits of foreign students that you mentioned.