I guess the point is that a rimmed, small shouldered case will last well if the headspacing is set to be correct (tight?) on the rim. Improved cases usually have larger (stronger) shoulders than the parent cases and headspacing on the shoulder instead of the rim will work to give good case life. There are exceptions where it doesn't help - soft brass (and maybe excessive firing pin protusion and strong springs?).

I would definately recommend rim headspacing on the 303 parent cartridge and the 35cal+ derivatives on the 303 case. I think the 27cal and below have a fairly decent shoulder and shoulder headspacing would be okay. I understand that quite a few people have 35 cal derivatives that do work with shoulder headspacing but this will not be the experience for everyone.