I render cell shaded for a number of reasons.

First of all, I've always wanted to make cartoons, but after years of trying on traditional 2d sfotware I finally came to the conclusion that I just can't draw :p

2nd, I use the cartoon with lines preview mode in poser 4, so it's a lot faster. It's nice to not have to wait until the end of the evening and then leave my project rendering overnight.

3rd, Pixar, Dreamworks, and all those other groups are very very good at what they do. So, I could either make what I think is a pretty cool looking cartoon, or make another really amature looking cgi animation. I'm not as good as them, and I don't have the time or resources needed to get to that level. So, I do things my own way.

My method is to animate the characters in poser over a static background image, either something I drew freehand, or a photograph or render from another program that I've messed with in photoshop. I'll render each character by themselves in cartoon mode and then again in alpha mode so I can composite it all together in aftereffects and add any additional effects or 2d animation elements that I need. It's working out pretty well for me, and my current project is finally in post production. Hopefully I'll have a new preview of it up soon.