I read your car insurance analogy further up the thread and I'm not convinced it holds water. It's more like an insurance company offering you insurance if you currently don't own a car but there's a slight possibility you might get one during the year; but you definitely can't have insurance if you already have a car even if you've owned one for 20 or more years and never claimed.
I read somewhere that about 500,000 houses are potentially at risk - I'd bet that the Chancel Check kicks up a much higher percentage of enquiries than that figure would suggest but, if the "true" risk of actually being "called upon" - even if you are "eligible" must still be sufficiently small else the TV adverts would be swarming with companies offering to take your money by now and every householder in the country would have been bombarded with junk mail.
A question I think that needs to be answered is what proportion of houses that are potentially at risk are actually liable. (The other question: Of those that are liable, what percentage will be "targetted", is, obviously un-answerable).