Very interesting points, Dean. I don't know how easy, or even possible, it would be to actually trade DVC via SFX, now that DVC has the new policy of not letting non-members confirm reservations with member services. However, I was able to join SFX as a DVC owner, and I feel that access to SFX's sell-off list is itself a very good benefit and makes it worth joining SFX if one owns at an eligible resort, even if one never trades via SFX.
Since SFX won't take DVC studios, I don't think it would usually be worthwhile to trade a DVC through them anyway; one-bedroom units cost just too many points to be worth trading.
As far as I can tell, SFX is letting me stay a member, even with the new DVC rules. I don't know if SFX hasn't heard about the rules, or knows about them and still wants DVC owners as SFX members. I think it would be poor public relations to revoke DVC owners' SFX memberships.