I guess dropping the saddle becomes less necessary if you’re riding low-tech XC. But for general trail or mountain biking it’s a necessary part of good bike handling. Ok maybe it wasn’t done 20 years ago but riding technique is far better understood these days.
I was taught it on a skills day by a professional coach.
If you just want to ride low-tech, then saddle up is ok but if you want to:
Pump
Manual
Front-wheel lift
Bunnyhop
Drop heels
Jump
Stand on the outside pedal in corners
Nail berms
Steep techy descents
Then saddle down is essential to get the technique right.
Riding actively, moving your bike around/moving yourself around the bike, you need saddle down and out of the way.
Obviously, back up again for the climbs