Well I ride Tennessee Walking Horses and the "Chair" seat is the correct seat....as long as you still have your back straight...and your legs are not too far infront of you. Everyone has thier own way of doing things, so each to thier own.
Chair seat in Saddle Seat is not a desired thing and if you are doing it you are incorrect, either with the definition of chair seat or with correct Saddle Seat Equitation. Sadly since many Saddle Seat riders think only Equitation riders should care about their form and many trainers don't think its important since they aren't Equitation instructors you will commonly see a chair seat in Saddle Seat. Saddle Seat riders should have a straight line from their ears to their shoulders to their hips and down to their heels and want to keep their toes behind their knees. More is explained on the Saddle Seat Equitation website: http://regaljada.tripod.com/saddleseatequitation/