Dunno what made me think of this just now. When I worked for IT in a school district way back in the 90s, a librarian told me she kept a supply of mouse balls in her desk because kids would steal them out of the school computers. What I remember about those balls was they picked up dust and crud off surfaces. Pretty soon optical mice came along and they were history.
Yea, eff those damn balls, and I had some nice mice back then.
I recently cleaned up several large boxes of cables, parts, etc. Found a track ball, but not a single ball mouse in there.