Dr. Albert Bartlett (Professor Emeritus, Physics Department, University of Colorado):
Can you think of any problem in any area of human endeavor on any scale, from microscopic to global, whose long-term solution is in any demonstrable way aided, assisted, or advanced by further increases in population, locally, nationally, or globally?
This seems pretty easy. Science, sports, construction. The more people we have, the more people we get working on these things.
A local increase in population (up to a certain maximum that I can't recall at the moment) supposedly makes the provision of city services more efficient per capita, though I'm just taking my Human Geography professor's word for this. More people in a region increases trade, which increases wealth for everyone.
Anyhow, this is the same guy who said "The greatest shortcoming of the human race is our inability to understand the exponential function.", which is the quote I really like. As far as I've seen, people often don't have good intuition about things governed by the exponential function.