You are right that Blender's user interface is definitely not beginner friendly. But once the learning curve is over, modeling becomes very quick. Also, the proposed CAD interface could be much more intuitive, and is probably the way most CNC operators would use the program.
Blender has built in support for animation, using either paths, keyframes, or motion curves. This should be more than adeqaute for cutting simulations, no matter how many axes are involved.
For example, the cutters path would naturally be guided by path animation, and cutter tilt or lathe rotation would be handled by motion curves.
I don't think that the simulation will be slow, because Blender is known for being one of the fastest pro 3d animation programs. Faster and lighter (less than 5mb download) than Maya, 3DSMax or Lightwave, which are some of the most popular proprietary animation softwares.
I think that Blender would be faster than MasterCAM, which is the only pro CAM software that I've had any experience with.
Blender doesn't need to have CAM scripts, as any CAD work could be exported, but that isn't very convenient. The cool thing with scripts is that you can run them one at a time, so you never have too much resources dedicated at once.
Anyways, I'll upload the CAD interface when I finish a few basic functions, so anyone who uses Blender can try it out.
In any case, there is a real need for a comprehensive opensource CAD/CAM solution.