'expensive 3d software' have gamma controls that look like this:

So it's not quite the same thing as poser's gamma correction. (Note how you can control the result in camera terms which is very important if you want to create photoreal work.) Since we don't have anything even close to this in poser, there's really no argument as to whether poser makes a decent gamma tool. Even tools in photoshop kick its ass in this area.

As for why it's a bad idea, regardless of app, it's mainly cause most renders need at least some postwork before they can be called 'final' and this has to be done before gamma correction. By rendering out with a gamma, you will have to first apply a 1 / 2.2 gamma to the image, then edit it, then apply a 2.2 gamma again. This will seriously degrade your render quality. Also you don't want to put a fixed gamma in your image either cause then you will have to do a rerender everytime you need to change displays. eg a 2.2 gamma works fine for online galleries, but not for macs or other things like printers, film etc. OTOH if you have a raw render without gamma, you can apply whatever gamma number you need to it.

The only reason to use render gamma is if you're just doing experiments and dont care about the work. Anything serious should be rendered to hdr and corrected in post.