Equally the user can't determine whether the site is of any value to them if they leave as soon as they see an advert. Some sites I go to are stores, who are trying to sell me stuff who also have auto playing adverts. Crazy but true.
I like what OkCupid does. If you have an ad blocker enabled, they explain in the space where an ad would normally be that while OkCupid is free, it runs on ad revenue, with a link to their donation page.