Well thats kind of a hard question to answer and it depends a lot on your current 3D ability vs. your current illustration ability.
Everything you see out of a 3D program (even the cell shaded look) starts as a model and ends with a render process. To create a "2D" animation in something like Max you still need to create 3d models, setup lights, cameras, and textures. Then you need to animate within the 3d software constraints and render the final.
Flash/Synfig/Toonboom etc are still based on traditional illustration techniques and require lots of hand animation (keyframe - tween - keyframe).
If your a decent illustrator you can create realistic shadows, motion etc in a vector animation app. However if your better at modeling the 3d program will create shadows in the correct places, allow motion blur, depth of field, etc in the render process meaning you don't have to know how to draw those things into your animation.
Just a heads up as well. In my experience cell shade rendering takes just as much if not more time/hardware power as more "realistic" rendering techniques.
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