So, I was taught, that yea keep your butt in your saddle but, when landing from a big or small jump, you want the horse to finish the jump and so when you "snap" or land in your saddle at the end the horse can't finish and it hurts/they will most likely drop the rail.
Naturally it depends on how fast you are going, want to go, or even how high your jumping if your doing that. The faster or higher you go the lighter the seat you want.
when your doing show jumping you wanna keep a light seat but when your just doing flat work keep a firm strong seat
In the moment I ride 1,20m and for my feeling it is hard for the horse to jump higher if I have my shoulders over the shoulders of my horse also I think I have no control over the horse...
I think it can depend on which discipline that you are training for. Your preferred way of riding is brilliant for Dressage but the being out of the seat is good training for XC or Showjumping as it's easier for the horse to move more openly. It's good to be able to do both. When you are training especially.
It really depends on the rider's preference on how they want to and like to ride, I do both, when my horse is rolling along on a good pace doing what is should I like to ride lightly in the saddle at a canter, and when the horse is getting away from me I like to "sit and drive" stretch down my legs and get what I want from my horse! :)