POST versus Ajax call

Now that most modern browsers support AJAX and client-side requests without performing a POST, what is the role of POST (form post)?

Are there situations or general rules when a POST will always be preferred to a XmlHttpRequest?

All that POST is doing is placing variable key value pairs inside the server head. The advantages of POST I can think of are large amounts of data and mobile browsers. Are there many others I'm missing?

N.B. I know you can perform POSTs with AJAX calls, I'm talking primarily about with a <form> tag