In my experience pair programming is good when it happens organically and bad when it is prescribed. The times I have seen it prescribed have usually been the result organizational or planning issue. "This feature isn't getting done fast enough, you two pair on it." Or, "I don't want to bring anything else into the sprint, you two pair on this story." In those cases it seems like leadership is looking at pair programming as an avenue for gaining some form of efficiency, but most of the time this encumbers things further.