To add to the anectdotal evidence. I applied to Harvard last admissions cycle before finding this forum. Had I studied this forum before sending out apps, I would have never applied in the first place - and I probably wouldn't have to reapply to Michigan after getting waitlisted. So even if being a GMATClub member doesn't improve your chances anywhere, GMATClub members are more likely to apply to schools that are a better fit. This would lead to applicants from GMATClub having a higher acceptance rate.
I think this makes a lot of sense. People who frequent the GMATClub forums are better informed and more realistic in their school choices. Being able to see the caliber of students that a certain school usually accepts helps us self-select into schools that fit our profiles.