Why Use Camera Mapper?

Why wouldn't I just slice a photo in Photoshop and import it into After Effects using Vanishing Point?

First of all, you have to own Photoshop Extended which is expensive while Camera Mapper is only $79. If you already own PSE then, as you know, you could use the Vanishing Point feature. However, do you really want to jump into another application and interrupt your workflow? Also, VP creates odd artifacts at intersections and the way it sets up a 3D scenes can be prone to error and unpredictable results when it comes time to animate. CM gives you full control over how you want to map your scene directly in After Effects. CM does not limit you to only using a still image as source; a locked or stabilized video shot can be mapped, bringing your scene to life. It's really quite fast and seamless using CM.

Incidentally, Aharon Rabinowitz put out a tutorial on VP that spends most of its time working with limitations of VPE:
http://cowcast.creativecow.net/after_effects/episodes/Vanishing_point_POD_iPod_Lo.mp4

Also, please remember that AE does not have a solid solution for projecting textures across multiple layers. In AE, setup time is a pain, and there is no way to project mattes which is very important. For example, if you have a photograph with some trees in the distance and want them projected onto a plane closer than the barn behind them, in AE you'd have to use their incredibly bad masking tools to Roto around the trees in the Comp window, then adjust this matte as the camera moves. With CM you can pull an extraction of the trees in Photoshop, then use this alpha on the projection and clip the geometry you are projecting onto.