So I noticed they make these roller nuts and bridges (tune-o-matic, in particular). Why doesn't everyone use them? Seems to me that they would have many advantages over their regular counterparts. Does anyone have something like that on their guitar(s)? Impressions?
Dear God, do you actually answer prayers?

Yes, but only in a way indistinguishable from random luck or the result of your own efforts.
Probably because people dont feel they need it, cant be bothered installing one, or dont know they exist. They're good, but pretty pointless if you dont need it.