I have a handful of times. They we're quite insightful and entertaining, though mostly not helpful since most of time they were commenting on things that just weren't meant to be. "It is possible to commit no mistakes and still lose. That is not a weakness; that is ~life~ interviewing."

Definitely a few times where I raised an eyebrow at obviously-not-HR-approved comments that made it through.

Only benefit these companies got from providing feedback was some word-of-mouth to my friends to interview there.

Really? I've rarely not gotten feedback on rejection - some companies outright issue it, though for most I have to ask.

On accepts never because people worry about awkward future colleague relationships.