Quote - They can take a normal snapshot with their digital camera, upload it to their computer (as a JPG image) and display it on their screen, and it looks fine.

Thats not really true. I remeber times as the photo hobbiests have published their gratefully and workfully done pictures in foto magazines. But since the digital revolution has invented telephones to make photographic pictures, the internet is overcrowded with rubbish images ... maybe (not really) technical rather well done but with senceless content.
But this is not the real point, see below ...

Quote - But they set up a simple scene in Poser and display the render on their computer screen, and it looks horrible. Why? Well, at least partly because their digital camera automatically corrects the image so that it will look right on a computer screen; but Poser does not.

This is the true difference. Out there in real world the nature is doing the lighting, the sun, the atmoshere, the clouds, and, and, and. You are seeing the result and if it's interesting for you, your camera is used to make a picture of that. It only depends on the technics of the camera how good the image will be. At that point GC and other things are touched.

If you work with poser the user has to do all that what's done normally by nature. So he has to ligthen the scene for his taste and, assumed his monitor is well calibrated, he should render finally if he's mostly satisfied with the artificial look and feel.

Quote - Many people increase the intensity of their lights to try to compensate for too-deep shadow areas in a non-GC'd image.

With look to every photoshop workshop, tutorial or also the gamma curve, GC doesn't influence darkest or brightest areas a lot. It's mostly working in the midrange. Can be seen in the demo pictures. It influences the desktop but not the candle flame nor any of the shadows at the wall or under the desk.

So with a look to all the smaller or bigger flaws poser is having with light and shadow, e.g. lights instead of shadow seen behind a dense wall, it's no solution to do any only technical aspect with single shaders. If it's of no artistic value it's a loss of ressource ... though a interesting scientific aspect.

As far as for example GC is not a general function of the program it's better done with programs developed for that, e.g. photoshop, psp, gimp, ... if neccessary.

Just my two cent ...