I've never once found a use for the exile button.
It has proved functionally easier in the past to either buy off inconvenient Dukes or else plan for the secession you'd face for banning them in advance, e.g. wait for them to be absent or get your military in their city and then ban them.
Historically the only way you could 'un-Duke' somebody would be to arrest them for treason, and even then you'd have to catch them off guard. You'd probably end up fighting over it a bit on the field anyhow.
