Use cases are up to your imagination, just for example, think of a html5 game where you want to highlight but skip elements, or move cursor forward and backward... only a trivial example
To support older browsers you could use something like xpaths.js but safari, chrome, firefox all support document.evaluate() and manipulating the element is a no brainer just do addClass().
https://github.com/andrejpavlovic/xpathjs
Despite being a jQuery fan (I've used it to build 4k line jquery chrome extension) I find it much easier to just look up the javascript equivalent, more than often, it's worth knowing since it frees you from using library.
But keep at it, I think maybe if you could build something that does a very good job of highlighting dom elements on mouseover/mouseout with some intelligent highlighting (simply attaching event handlers to all elements on the dom page produces a jerky) I could see that being quite useful.
All in all, take my view with a grain of salt, just because I failed to see a use case and think it's more cost effective to know the DOM manipulations with javascript directly, does not mean that somebody out there might justify it's use.
Good stuff!
We did not find anything and we thought of writing it directly.
Obviously it's open source so anyone can use it and do and suggest changes, we are curious to see even the possible use cases that others have.
Probably using what you say would be the best is not easy to say so on the spot, it should be seen in any detail.