Sticky has more uses than just headers, for instance sidebars and other sorts of control panels can make use of it to improve usability as well. It's a usability feature, not a fad design feature.
You mean like in the bootstrap documentation, where with a small enough viewport you cannot access bottom of the menu?
That's a failure of the implementation, not a failure of the concept. Having dealt with implementing things like that in the past, I can't offer an excuse other than laziness for those implementations. However, it doesn't take away from the concept.