I think it's a very serious issue. Clearly the population will balance itself at some point, whether by education/moderation or mass-starvation. However, I believe we have a finite window of opportunity to make a number of critical scientific and social advancements in order to continue our species' progress in the next few centuries and beyond. This window is defined primarily by the availability of materials and energy which are necessary to support technological development at our current scientific frontiers. Increasing population, especially in the developed world, consumes resources faster and thus shortens the span before many critical materials become scarce. Better extraction and synthesis will likely become available in the future, but first we need to get there.

I am not arguing that we shouldn't have children at all, but responsibly limiting our population seems like a critical supporting factor in any qualitative advancement we want to make as a species. This is especially true since much of the world doesn't seem likely to do the same any time soon.