I'm not quite sure how I'd use the metric or if I'd use it to purge notes.
Searchability or recall can be a problem sometimes though, so "searches where I had no results or didn't visit anything" could be interesting.
Especially if I try searching later and find a note answering my question with bad "SEO".
Another idea I had was to make a Firefox extension that searches my notes and displays results before search engines since I reflexively search things in browser sometimes.
I just, again, for me personally, gain a lot of value from writing the notes even that I never go back to revisit.
So for me personally, the metric is interesting, like I said, but doesn't capture "how useful" a note is, because I have most of my utility outside of that use case.
1. I have it committed to memory. 2. I never needed it.
I have no way to discern whether either (1) or (2) will happen as far into the future as you care to specify; so it's mostly a moot point. In any case, I will sometimes just do random walks through my notes, wikipedia style, and find a lot of value in it.