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.
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! :)