You use the best tool available for the task at hand. If you've got access to an airbrush, or can afford to buy one, why not use it when the situation calls for it? Would you cut down a tree with an axe, desipite having a chainsaw in the garage, because it's more "hardcore"? I wouldn't give any extra points to someone refusing to use an airbrush, despite having access to one, and put in hours and hours of work doing something with a regular brush that would take 10 minutes to do with an airbrush. I would just think it was a rather silly decision.
As most have already said, airbrushing is a skill in itself. It's not simply point and press and voilà... instant masterpiece! It takes time and practice to master, and, as Hendarion explained very well, there are a lot of things that can go wrong BECAUSE of the airbrush. Most people I know, including myself, actually were reluctant after using an airbrush the first couple of times because it was tricky to handle and the process felt more technical than the sort of casual, relaxed feeling you get when painting with regular brush. But when you learn how to handle the airbrush, and learn to control it, you get the same kind of relaxed feeling.
There are a huge number of tools and aids available for us to use in this hobby: various paint brands, different brushes, sculpting tools, sanding tools, knives, drills, paint additives, etc. etc. They are a great help to the hobbyist for achieving certain things. An airbrush is just one of those, that makes painting certain things easier and quicker. Other things require other tools and methods to paint.
And, every single spot on any figure of any size I paint get some attention with a regular brush after the airbrushing is done. Airbrushing, unless you're insanely good at using one, can easily produce rather sterile results, IMO. Blends are nice, of course, but the whole thing feels a bit lifeless (I often think the same about 2D art done with airbrush). Therefore, I go over every surface with a lot of glazes to give them more life and a less uniform look. I often enhance highlights, as well, with a regular brush. And of course, I paint details and textures and stuff with regular brush. Not because it's more proper, but because I find it to be a better tool for the task.
Sorry for being long-winded, but the idea that airbrushing would be cheating always gets me going... So much, even, that I decided to end my lengthy absence from CMoN because of it...
