Are you talking about long hills or short hills? If you are riding with a group and you sit on a short hill while everyone else stands, you will get dropped almost every time and catching back up can be very difficult.

On a long hill, you need to pace yourself so you don't blow up. Even the best climbers will usually sit some of the time, but standing some of the time will let you use different muscles. Sitting on long climbs also builds pressure on your rear end and standing, if just for 10 seconds, can help to relieve that.