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.
I was in highschool at this point and I totally would have ratted any kid out for that.
No mouse balls would mean no Quake or StarCraft in the lab after school… Unacceptable!