I wouldn't say that stats have to be public in the sense of available via the website, but I would say that you should keep stats for traffic for a couple of years if possible. We found when we were doing research on use of DH resources for the LAIRAH project that it could be very hard to get access to log data. So we recommended that projects should keep raw logs if possible, or if not traffic reports, so that if they are requested by researchers they may be studied. I think that this answers the question of being accountable for public funding, without placing too huge a burden on projects themselves.