what if you've lowered your saddle for a descent... THEN what do you do?
I have a Rock Shox hydrolic seat post. I hit the button =).

Without a dropper seat post, you'll have to make compromises. I can confirm that it IS easier with the seat at the optimal height for climbing. I also find that I don't really have to drop my seat for decents unless there are ledges of 2 feet or more. I can get my weight far enough back on the bike for smaller ledges and sustained grades without having to drop my seat from climbing height.

When I began riding, many people told me to get all the way behind my seat for every little thing going downhill. Gene taught me otherwise. It's really about weight balance/management. If you watch downhill racers and even XC racers, they only get behind the saddle, with the saddle level with their torso for the steepest of gnarly rocky decents. Being WAY behind the seat puts most people, in a position where their arms have no bend left. Bad idea to be at the "end of your rope" like that.