If you've been a website owner for long, you know that Google loves changing things up (some might call it “pulling the rug out from under you”).
Take for instance how Google decided to yank the Google Authorship photo from their search results. Many (including yours truly) spent significant time and energy making sure our Google Authorship was in place … and now those pictures are gone. The time wasn't fully wasted as they still see authorship as important, but it's certainly not as beneficial in the short term as many people were hoping for.
Well last week Google did it again. They announced that SSL is now a ranking factor for websites. What does that mean exactly? Well, if your site is served up as 100% HTTPS then you will get a bit of a ranking priority. Google calls this priority a “very lightweight [ranking] signal,” but it's one that “over time, [they] may decide to strengthen.” This means that there are a lot of SEO strategies such as producing high quality content that are still more important for rankings.
While this news might be a bit exhausting (because now you have to jump through all the hoops again and convert your sites to HTTPS), I think this is generally a positive change and one that Google is unlikely to back off on later.
Now as for membership site owners, I think this change is definitely going to be much more important for your members and a bit easier to implement than for the vast majority of sites on the web.
WordPress membership sites should not only be running their payment pages over SSL but also their members-only protected areas. Many membership sites protect premium content that you wouldn't want accessed publicly and in addition to that, many membership sites contain protected areas where private user information is recorded. Having the extra security of HTTPS will give your users more confidence that their information is safe.
Really though, I think this could be a modest opportunity for you membership site owners to help your rankings. Even though it's a very small ranking signal now, it's still a ranking signal, and one that is very straight forward to implement. Although it can be some work to get SSL working effectively (and certainly not the most important ranking strategy), unlike many other ranking strategies that are ambiguous, this could be some low hanging fruit to be harvested.