I know that DL never comes on board to UG someone (I don't think this is "new" from what I have read on the DL forum).
I have been pulled up to F twice in the past month. First, on JFK-SJU (1/15) and then JFK-LAS (2/12). Both flights were full with a large standby list, so that may have been the reason, but I definitely wouldn't say it never happens.