Very good point. On plus side of useability for always-post, you get all the data they are currently working on.
Imagine a page with navigation bar at the top and a user is filling out the form. Even the navigation bar secretly uses POST buttons/links. Since we always POST, we even get their partially filled form so that when they click 'Home' or something we can ask 'Are you sure you want to leave the form half-completed' and keep all their data and everything.
But the back button thing does freak people out.