The worry is that depending on foreign volunteers to do the manual labor is a short-term fix that creates a culture of dependency. By contrast, training skilled workers (might be) a long-term structural fix. But this essentially limits the useful on-the-ground volunteers to those able to teach skills, rather than those willing to do unskilled labor.

I think this piece (plus the excellent comments) is a good summary of both sides of the argument: http://mitpsc.mit.edu/globalchallenge/?p=335