True, and that's why active managment on the part of the webmaster is required. If your website is a large forum that only covers one topic or a huge encycolpedia that covers everything, it may not matter. But if you have several discrete topics, they are bound to show some seasonality, due to holidays, school years, the construction cycle, etc.
The trick is more deciding when to go live again than when to take ads off. We count on past performance for that.