I too was always told that English saddle pads were only used to keep a well fitting saddle clean. Of course I think I might use a thick pad with wedges if I had to use a saddle that didn't have optimum fit.
I have a friend that makes and fits saddles. He told me that putting a thick saddle pad under a well fitting saddle is "like ordering a custom pair of boots and then wearing one or two huge thick pairs of woolly socks with them, it totally throws off the fit."
He also says that many of the "close contact" saddles that are so popular with riders today do not offer enough padding in the panels for sensitive horses spine and back". The result of this is having to use a thick pad which renders useless the fact that the saddle has limited flocking in the panels for closer contact with the horse.
I always wonder if that is why people even use the half pads?? I've never had one except when I needed wither relief on one saddle (didn't really work like I had hoped ). I always just use a thin quilted cheapo pad to keep my saddle clean.
I DID recently buy a saddle about 10 years old and It was obvious that someone had ridden with a half pad only. The panels were spotless and the inner flaps were badly degraded from horse sweat. The leather just crumbles to pieces after a few rides. It was so damaged there was/is no saving it. I'll have to take it in and get the inner flaps completely removed and replaced on both sides.