The advantage over preview is they are much larger and you can resize cover-flow to your liking. So you can easily flip through them and much better than preview in that fashion.
Once cover-flow has built the "preview" image for a given file, then it doesn't have to generate it again.
Yes, I agree, if I could reach up with my finger and flip through it like on the iPhone, that would be awesome and yes, cover-flow is not the same using your mouse vs your finger.
Obviously, it will have very specific useful purposes, but outside that, your probably not going to use it. I know I won't be flipping through my Applications to figure out which one I want to launch today.
