Quoting Ellery:
central notice manages a cookie called 'centralnotice_bannercount_fr12'. It simply counts how many banners the client has seen. I would like for the content of this cookie to be added to the x_analytics field.
With this information we can:
1. Increase the accuracy of our testing by controlling some of the confound that comes from treatment groups with different banner impression histories
2. Determine the optimum number of banners to show people. i.e. "its worth showing up to 30 instead of 5 impressions like we previously thought", or "after 5 impressions, the probability of getting a donation is so low that we should stop compromising the user experience with banners".
Both of this are pretty important for "baselining" this years fundraiser. I don't see any privacy concerns (its just a count, much less specific than for example the referer or ip/user_agent)
It would suffice to send it back only on webrequests where the uri_path=/wiki/Special:RecordImpression, but this may be harder to do than sending it back on every request.