I find standing on super long climbs is a tremendous help. It works different muscles and gives you a real break - also good on the undercarriage as Nanci points out. It gives your upper body a nice break, too, working idfferent muscles there as well. You wouldn't think that was important, but for a super long ride it really is.

One thing I really worked on last year for death ride training was the right kind of standing for long climbs. I had always stood to power over small rises without shifting, etc., so the standing involved a power increase and speeding up. Well, for a longer ride, you need the standing where your speed doesn't increase at all. That's a great trick to work on and it really comes in handy for all day/all night riding with long sustained climbs.

I think standing on a MTB (on dirt) is tough because of the traction issues. But I tend to think everything on a MTB is tougher because I hardly ever ride them anymore.