some of the practical advantages of spirals, in terms of physics, have already been mentioned.
i'd add that, in my understanding, even if this has to be "learned", it IMO is the natural way of movement.
http://www.alexandercenter.com/dartspirals.htmlthis article was shared with me many years ago by steve james. it's not about MA, but i think the understanding of anatomy still applies.
i think everything does/can/should, include this.
spirals, as you point out, can be large, or tightly compressed. in which case a seemingly "linear" movement can express, and its effect can be greatly increased by, spiral movement which is not at all obvious to the casual observer.
to try to answer your second question, indirectly: from my POV, just about anything can include the element of spiral movement.
the tcc i follow is basically yang style. some would say yang does not include spiral movement. but my understanding is that it does, intrinsically.
i'm sure, from reading your posts, and seeing some of your clips, that you already understand this, but: su dong chen told me that he considers the "essence" of bagua to be spiral movement.
best...