up vs down
mentally picture a mountain bike with the handlebar exactly the same height as the saddle. now picture that bike pointed downhill on a steep slope. the bars are now much lower than the saddle because the front wheel is further down the hill. that's why downhill and freeride bikes have higher bars.
conversely picture the same mountain bike on a steep climb. now the handlebar is much higher than the saddle. that makes it harder to keep enough weight on the front wheel to maintain control when pedaling slow and steep.
for climbing, a lower bar position works best. for descending, a higher position is preferred.
I find bikes with relatively low bars go downhill a lot better than bikes with high bars -- higher than the saddle -- go uphill.
that answers the bar position Q. Now about saddle height:
I think dhers use a lower saddle position -- not talking relative to the bar, but relative to their leg length -- for a lower center of gravity and so they can get way back more easily. since their pedaling is more about maintaining momentum and less about powering up steep climbs, a lower position works for them.
For general riding that includes climbing, you need to discover your perfect saddle height. It'll be higher than the dhers use for sure. good chance it's also higher than you're accustomed to, higher than you're now riding. I see a few folks with saddles too high, I see tons of riders with their saddles too low.
it's a myth that mtbers need a lower saddle than roadies. veteran roadies can set their saddles perfectly, and can detect a change of 1mm -- the difference between thin and thicker socks, or thin and thicker chamois pads. I know I can.
All formulas for saddle height will keep you from destroying your knees with a grossly low or high saddle. NONE of them will get you at exactly the right sweet spot, which for most riders is only a range of a few millimeters. mark your seatpost with a piece of tape, so whilst fiddling with it, you can tell if it slips down. then start raising it 1mm at a time, then ride awhile. keep doing that until it definitely feels too high. you'll know when you can's spin smooth without throwing your pelvis into it and/or without starting to point your toes to reach for the pedals. at that point, come back down maybe 2mm and try that. you'll definitely be within a couple of millimeters.
too low and you'll unconsciously be pointing your toes upward to compensate.
too low=easy to spin really fast, but you'll lack power to the pedals when pushing a big gear or when climbing.
slightly high=you'll have good pedal power because you're pointing your toes and using your calf muscles to supplement your thighs and gluts, but your oxygen use goes way up as a result so you get winded on climbs and when pushing a big gear (moreso than you would otherwise).
once you experience the Perfect saddle height, you'll when you got it and when you don't.
always mark your posts with a piece of tape, some folks inscribe a scratch mark but since saddle height is a changing fluid thing over time, you'll end up with a lot of scratch marks eventually. that way if the post slips in the frame even a little you'll see it. otherwise, the body being so adaptable, you end up struggling because the post slipped down but you just think you're having a bad day.
about Perfect seat height being fluid and changing: I can dial in a bike so the height is Perfect. Then ride other bikes, I got lots, and come back 6 months later or next season, jump on the Perfect bike and the height will be off, despite the same shoes, same pedals, same socks, same shorts.
Stuff like that used to drive Eddy Merckx nuts. He was well known for fiddling with his seat height with an allen wrench While Racing. Campy developed its first seatpost quick release, heavay burly thing with a big fat diameter to replace the fat male/female allen head seatpost bolts used on frames back when frames had integral seat binder and not separate removable BMX style clamps like now.