2 interesting observations about your latest post. First, I also saw CSNY about six years ago.... It was at Madision Square Garden, though I'm sure it was part of a tour so it would be even more of a coincidence if you saw them there as well.
Second, I recently saw Neil Young in an interview where he said that when he plays a live show and someone (primarily the press) complains he didn't play such-and-such a song, he thinks to himself that's a good thing! Because he feels what gets played live should be a reflection of how an artist feels he wants to express himself at that particular time. So playing certain songs all the time is sort of cheating the audience because it's not where the musician is at that point. He had similar remarks about his studio work as well.
So I found it interesting that you mentioned your disappointment.
What song was it, anyway?