There are pros and cons about this subject pertaining to illustrations and toons and entire subject of realism and realistic rendering.  I tend to lean towards trying to strive for some sort of realism.  However, I believe that the technical aspect of Gc is to correct a problem that has been around for a long time.  You can get a better definition about gamma correction from wikipedia, so I won't attempt to explain the concept here.  And Bagginsbill can give much better insight than anyone else. 

But in the case of a new user, taking Poser right out of the box and rendering their first image, and finding that mental image is nothing what is represented on the monitor, it can be quite disappointing and disharkening.  And that is because the image is processed linearly, without any correction factor and appears too dark.  Then the user goes back and begins adding lights to brighten the scene.  And then more lights until the render engine quits in mid process because there is too much data to process. 

Now for an illustration or toon, the image is taken into post production where all the corrections can be made to convert it into it's final form.  But, how much postwork are you willing to invest into a bad render to begin with.  If it is terrible, probably no time at all. 

The purpose of Gc is to render the scene correctly the first time (well, maybe not quite the first time) with a minimum technical wizardry and at the quickest convenience.  For example, at one time, users would create multiple light sets in Poser, which are tremendous resourse hogs, to fill every shadow in a daytime scene.  Then wait for hours to render.  Now, we can recreate that same scene with Gc, on a similar computer, with 2 lights and render in several minutes.  A time saver if you are crunching illustrations for a story.  If you can technically do it correctly the first time then why not.

I use gamma correction.  I can recognize it when someone does not use it.  The specular bloom in the texture, over lighting in the scene, fine details that are washed out, and deep shadows that are just too deep to perceive any detail at all.  These are qualities which can be found the realistic realm, but if you want to carryover some of those qualities onto the next level of your work, would you rather have it correctly available from the beginning than trying to change it in postwork?