Well, my experience is that of the OP's. I stand to stretch for a few seconds on long rides, or to just move around, but never while climbing. Just the act of standing hurts like hell, and I spin up anything. I may be going slowly, but I can climb steep grades. When I am riding behind DH, I always know when he will stand. If I stand, all that happens is my speed goes down as he accelerates. It just hurts all over, so I stopped trying. I approach it in a similar way as Marni described.