Could you show a little example ?

Usage in the autocomplete plugin:

function state() {
  return $("#state").val();
}
$(...).autocomplete("county.php", {
  extraParams: {
    "state": state
  }
});

Part of the implemenetation:

var extraParams = {
	timestamp: +new Date()
};
$.each(options.extraParams, function(key, param) {
	extraParams[key] = typeof param == "function" ? param() : param;
});

Implemenetation, if the feature would be added to core, would just pass through extraParams to $.ajax instead of "parsing" it itself.

While this isn't of much use for direct usage with $.ajax, it makes sense for $.ajaxSetup, which you call at some point before starting a request.

Having the same parsing for the actual data parameter would be useful too.

Fixed in [5478].