Yeah, I'm sure they bring a lot of demand for goods and such, but as a blue-collar worker all I really know is that they depress my wages.
That is one thing the authors of these studies don't have to worry about, right? They won't depress their own wages.
There are many other factors to keep in mind as well, the cost to the health care system, and school system. (penal system too)
This is far from a 'black and white' argument.