Using physics in interfaces gives naturalistic motions right out of the box, you get nice animation curves for free essentially. Of course if you want flat pages, then this doesn't matter, and I do agree with providing flat versions of useful stuff, but if you want something that has a good feel for any gui animation, then integrating physics makes a lot of sense. Not so that you notice it, but rather so that you don't.
Even with flat UI. The motion of natural 'objects' is very much wired into us. If you have elements moving in a 2D plane, their motion is typically perceived as more 'pleasing' when they obey physics often at a subconscious level. Its a bit like the golden ratio.
