Good point...
In a CNC application, I'm assuming the motor won't be called upon for full power on a continuous basis. I'm thinking that certain activities (e.g. getting the x-axis to move from a dead stop, or lifting the z-axis, etc.) will be demanding on the motor. Milling or routing any significant depth, at any significant speed will also be demanding.
So, (I think) the idea is to hit your target spec, but have around 2x headroom (someone else mentioned 3x...but that gets really expensive). In my example, I'd say that I wouldn't want the nominal linear motion power demand at the cutting bit to be any more than around 180-185 N of force.
I'm just getting on board with this stuff, so please (anyone) correct me if I'm wrong.
Thanks,
Chris