I think the main advantage of OW (from the passenger perspective) is the greater consistency of the products. I have the impression that OW is quite demanding when admitting new members. Also recent additions like RJ are not much behind their legacy members (some in this forum even argue they are better).
In *A the large number of additions in the last years has compromised the service consistency quite a bit. Some member *A airlines still do not offer a decent website in English (e.g. FM, AI), others are sub-standard in some products (e.g. long-haul C on MS). Obviously the other side of the coin is the number of destinations/routes added by this new members.