I think the biggest selling point of SL is you get lounge access and J class priority tags even when flying Y. Definitely more generous than the equivalent tier of some other airlines (namely SQ). Having been both SL and GO before, I can go as far as saying the drop-off isn't that big apart from slightly better op-up chances as a GO. The benefit of bringing a "plus one" into lounges is only useful if you frequently travel with non-status pax anyway.