chestnuthunter, when you post your opinions/preferences on a public message board, be prepared to receive comments on them whether they pertain to the OP or not. Rule #1 of posting on public forums. If you don’t like it, then you should not be posting. And no, I am not being nasty, I’m honestly trying to be helpful here.
Next, custom fit saddles that are fit just to the horse’s back are absolutely designed to be all the padding you need. Throwing a half pad under a perfectly fitted saddle only throws off the fit and can create problems, despite your best intentions. If you want a saddle to fit perfectly with a half pad, then it needs to be fitted/flocked by a professional with the half pad in mind. Just because you see half pads at shows doesn’t mean anything except they’re fashionable. I’m sure some of those people do not have a solid grasp on saddle fit, and probably some may be using those half pads when in fact the saddle fits just fine without it. Don’t just do things because everyone else does it. This is part of your education as a horseman. People need to understand WHY things are done a certain way, not just do things because everyone else does.
And no offense to your mother (her nationality has absolutely no bearing on this discussion), unless she is a saddle fitter and has personally fit those saddles to your horse with the half pad in the equation, then she is merely telling you HER preference, which may or may not be founded on proper knowledge. I would agree with her to use a half pad with the saddle that’s not custom fit. The custom fit one, however? That you need to contact your saddle fitter about, because the recommendation may very well be to use it with just a square pad underneath.