That's definitely an exponential distribution. I'm a data scientist and I keep going back to a few key notes over and over and adding little bits. I tried to track things like ideas, tasks, and just journaling, but it just didn't fit right. It's unbeatable for my work related stuff though, and the key things for me were mathjax support and being able to paste screenshots directly onto the page. Fantastic product.
I agree there is value in just creating the note and distilling your thoughts even if you never revisit.
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.
Oh, I 100% agree that there would be some kind of exponential falloff in which notes I go back to.
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.
