Yes, If you want Signature benefits, which includes early access to book, upgrade to next size unit, and ability to trade into RCI/Registry collection, you have to tier up and own in Signature for it to even be worth it.
Otherwise, I think if you just own UDI or Signature wk at specific location, then that's all you get usage for. Unless you converted to pts system, hence status tier. That old thread did mention something about the SBR UDI pts being restricted to just SBR. So if u found a seller and it's a lot of pts, may work out if u like SBR.
IMO, the pts value isn't worth it compared w reg units pts. If u want real luxury w concierge service, go with a higher end brand other than HICV.