It's an option is all, some people can churn out sprites like nobodies business - but it depends on the quality you are going for and the number of sprite variations you need.
Typically though, it's much easier to animate on a 3D model then convert to 2D, for one thing it's tricky to even work out 2D animation without a lot of training, practice, and a helluva lot of skill. A lot of 2D artists will mirror sprites and skimp on animation, it's simply crazy to want to draw 16 frames of animation for 8 directions for 1 character walking. It's really, in my opinion the difference between doing 1 whole character sprite sheet in an evening instead of a week.
If you can draw it in 2D, you'll be able to model it in 3D without too much trouble and learning. Really, modelling took over 2D for me within about a week of just messing around. Rendered is quite a destinctive look though, sharper and more detailed than most artists would draw in 2D. You can always tweek the renders with whatever style you are going for of course, adding thick lines and stuff. You'll know yourself what sort of look you want, but even if you just need a start, like flat colour that you can draw over - that takes very little time and gives you full control over the looks. South Park is made in Maya, a traditional 3D modelling and animation package - so even if it's all 2D, 3D animation can still save a lot of time and effort.
So really it depends on what look you want - if you are making a mario platform game, then 2D is probably the easiest. If you were making say, an isometric game like Diablo, then rendered models is certainly the way to go. You could grab some FPSC media from the store, render it to 2D, and have an incredible sprite sheet if you wanted.