Eventually, you are going to want to be good at climbing out of the saddle. Yes, it is very fatiguing at a high cadence (90-100), but I can think of at least three circumstances when you may want to employ it. First, on short hills I find I can often maintain bike speed by getting off the saddle in the gear I'm in on the flat and climbing at a very high output (e.g., 750w) for ~10-15 secs. Second, on long climbs I find that I can rest up a bit by getting off the saddle every 10 minutes or so because it uses different muscles and I can rest my back. Third, I recently had to get off the saddle >200w due to an injury (strained gluteus medius muscles), but I was able to keep riding and even doing intervals for about 2 months while they healed.
The trick is to develop a good off-the-saddle technique. It's all about rhythm, balance and timing. I think it's closer to a dance move than a cycling technique. I try to take full advantage of my body weight on each downstroke and I try to minimize the use of my upper body. I see some guys really using a lot of upper body strength throwing their bikes back and forth. I try to control my bike tilt with my legs and use my upper body very little. But, this takes some practice to learn.
