I'm not a great biker/cyclist, but I'm a fairly experienced marathoner who enjoys riding hard on my mtn bike whenever I can (not often enough).
I find that I can climb better in the standing position, because it's closer to the "running" upright position. Many of my colleagues inform me that I'm using bad form and will never make it all the way up the mountain standing upright, but I seem to be able to "brute force" it with my aerobic ability and I often go standing for 5-6 minute stretches at a time until it flattens out.
Interestingly, I've tried the exact same climbs in a seated position, and I have no chance. My quads simply give out too quickly. I suspect this has everything to do with the fact that I don't have the quads of a true cyclist, but there's no doubt that as a runner, I can do longer steep climbs by staying out of the saddle.
That said, I'm working hard now to build up those quads so I won't have to stand all the time and waste energy - it does seem inefficient in the long run.