courier - 8:03 am on Oct 18, 2011 (gmt 0)
I am likely wrong, but could it not be a case of
1. If a +1 button is on your site and there are little to no clicks on it, would Google not be saying this is not a very popular site, and use it for a reason to down rank you.
2. If there is no +1 button to click on a site there is no info so Google cannot see how popular or un-popular your sites are, thus perhaps not using lack of +1 clicks as a reason to down rank?
Just thinking out loud!