Also: sometimes a site is misbehaving (for myself, or maybe for a user we're helping) and it's helpful to directly navigate to /logout just to know everyone is rowing in the same direction.
Using a POST, especially if you're building through a framework that automatically applies CSRF to your forms, forecloses this possibility (unless you maintain a separate secret GET-supporting logout endpoint, I guess).
When I originally started my community site I used GET for logout. However, users started trolling each other by posting links to log people out. It wasn't easy to control, because a user could post a link to a completely different site, which would then redirect to the logout link. So, I switched to POST with CSRF and never had another issue.
Using a POST, especially if you're building through a framework that automatically applies CSRF to your forms, forecloses this possibility (unless you maintain a separate secret GET-supporting logout endpoint, I guess).