Agreed. Spigot rings do not transfer any load. If they do it's minimal. They only help centre the wheel during installation. Many are plastic, but even the aluminium ones are relatively soft and not capable of transmitting any reasonable load into the hub. The wheel hub spigot is itself not actually that close a tolerance to the centrebore. It might really feel like it when you're trying, and cursing, to get the wheel on, but it's function is to prevent you from knackering the threads on the studs. From an engineering perspective it's actually a really sloppy fit. It's not even close to a press or interference fit as is the case for example with the hub bearing. So, no transverse loads are transferred through the hub spigot in normal conditions. There are tens of thousands of cars on the road using expensive aftermarket wheels with oversized CB supplied by the likes of Europarts, Halfords etc.. along with their spigot rings. So not illegal. Having said that, if I had aftermarket alloys with oversized centre bore I would make a point of declaring it to my insurance co. as a modification. To sum up, removal of doubt for the sake of a few quid is a good thing so best to go for the OEM spare. And as you say, in the event of a huge trauma to the road wheel, where one or more studs is not torqued properly ( and who really uses a torque wrench when changing wheels ), the wheel spigot could just be the last line of defence. I used to run an old Hundai Santa Fe and one day did'nt tighten up the bolts propertly. Heading down to Montrose I felt and hear a clattering noise. The bolts were finger tight ( doh! doh ! DOH ! ) and had loosened further with the vibration. The correctly sized (OEM) spigot/CB probably saved me from losing a wheel!. I'd had several goes at unsticking a siezed caliper and had the wheel on/off several million times and being totally hacked off with the rain, dark, frustration, blowtorch burns and skinned knuckles forgot to tighten up properly.