In ActionScript, like in JavaScript, there is an implicit arguments variable holding all formal parameters. The difference is that in ActionScript, this variable holds a real Array, not an indexed Object like in JavaScript. Like for the ... (rest) parameter, Jangaroo should add conversion code automatically.
Example:

public function argumentsToString(a:String, b:String):String {
  return arguments.join(",");
}

works in ActionScript, but not in Jangaroo, as the join method is not defined on the JavaScript arguments object.
Also, when using arguments as a .... (rest) parameter, Jangaroo optimizes away the Array conversion code, resulting in the same problem:

public function argumentsToString(...args):String {
  return args.join(",");
}

works, but

public function argumentsToString(...arguments):String {
  return arguments.join(",");
}

does not. :-(