Comments by Ashley77 (Top 1 by date)

The MSDN link you provided states "This attribute should never be set to false in a production Web site, even if the application or page does not use view state. The view state MAC helps ensure the security of other ASP.NET functions in addition to view state."

In short, if you disable ViewStateMAC you can no longer trust anything that is sent via POST, which includes all page controls including those that are not visible.