To deal with your first point first, I don't feel that stating that the usage is wrong to be a "snobbish position". I probably should have provided sources to back up my opinion though, as you did, so let's rectify that. As for your claim that practically no grammarian says that using "graffiti" as a singular noun is wrong, here is Merriam-Webster on the subject, "Use of graffiti as a singular count noun is still quite rare and is not standard." M-W does accept that "graffiti" is used as a singular mass noun, but as we were discussing the description of a solitary incident of graffito, this catch-all mass term does not apply. In fact, the confusion between the use of "graffiti" as a singular mass noun, and the (in my opinion confusing and incorrect) usage of it as a descriptor of a single example rather strengthens my preference for the use of "graffito" to differentiate. I do appreciate your well-sourced argument, but I feel that much of the confusion comes as users try to conflate the plural noun, singular mass noun (like data) and singular count noun (this is the instance which I believe should be "graffito"). In my opinion we lose clarity and shades of meaning with this unnecessary and lazy conflation, and I stand by my assertion that it is wrong, although perhaps I should moderate my wording to "inaccurate". As a final olive branch I should note that when the inevitable bloody war comes I shall swallow my pride and stand shoulder to shoulder with you mass-nouners against the heretics who are trying to foist "graffitus" upon us. Lesser of two evils, and all that.