Post mortems are generally worth the time for me, either for receivng good info or giving something of worth.
One rare type of post mortem I experienced at a local chess club was with one guy I beat fairly handily. He kept saying What would you do if I did this. I told him that I didn't want to play a whole other game. He seemed intent on somehow turning that loss into a win.
More common and more distressing is when during the post mortem it becomes very clear that you saw much more than your opponent, even when you've lost. Especially when I've lost I say to myself How can you lose to this idiot? a la Nimzovich.
I've got to be realistic though and acknowledge that I must be a dumbass in turn.
Such a post mortem can really be a psychic drain.
I can see it both ways. Most of the time, I like doing a post-mortem but would understand if my opponent wouldn't be open to such. I too would sometimes just rather play another, and save the analysis of the game for myself later.
To each his own!
~Jerry~