I want to give the user as much flexibility as possible regarding custom effects. Right now, you do have to edit the content pipeline code or animator source code to use your own effect, but in the very near future it will be very easy to do this without editing the code. Perhaps even as easy as just setting the effects in the model.

It is necessary for the animator to use the world transform, because it uses this as part of the animation. In order for it to be able to animate non-skinned models, which use BasicEffect, using the World parameter is the only option.

However, the View and Projection matrices are not needed at all by the animation code, and putting them inside the Animator class would place unneccessary restrictions on the effect code.

Also, look for a tutorial on how to make amd use your own effect in the near future. The actual animation code in the .fx file is actually quite simple.