Interesting notes. Touching on the most pertinent points to our discussion:

• It would seem that publishing the group's membership is, in fact, more of a PR/marketing gesture than anything else. There may be other reasons, but the general idea is to give people in that population a belief that, as you put it, "they are not alone."
• Population figures aren't a good barometer of, shall we say, "religious efficacy," but, you suggest, comparison of "conversion" rates to "excommunication" rates is. Would you include non-attending members in those numbers, or leave them out completely? More importantly, for this discussion, would such a ratio of "satisfaction" be something that could be contractually set forth: "X Church promises to Y initiate that Z services will lead to some degree of spiritual fulfillment" (or your money back, perhaps)?
• Is it cynical to consider that a legal document, if enforceable at all, would be of more value to an initiate (or his/her church) than the unwritten commitment to that person's chosen object of worship? In other words, is such a contract a violation of Moses' first commandment (Ex. 20:3)?