Originally Posted by Palomine
...A chair seat is never correct though. Good instructors never teach, or allow that. Good riders don't use one either....
A chair seat is fine for many things.
If someone has tight hips - and that is common if you take up riding as an adult - then a chair seat allows one to ride with a relaxed leg. I think a relaxed leg is more helpful than heels under hip.
It also allows a person with tight legs to ride with heels down. It offers a very slight disadvantage if the horse spins around, but is more stable if the horse suddenly stops. I find it easier to sit the trot in a chair seat, although that may be related to my tight hips.
It was the normal way of western riding (and English outside of dressage and jumping) until around the 60s.
Drawbacks are that it makes posting awkward, and it doesn't give you their best speed.
It is still pretty normal for cutting cattle, roping, reining and trail riding. If you expect sudden stops, then a chair seat gives better balance.
It isn't always right, but it certainly is not always wrong, either. I like English 1" leathers on my Aussie-style saddle in part because it makes it easy for me to switch from chair seat to forward seat, or to heels under hip, depending on what I want to do in the immediate future.