Sometimes you have quite a few places using the same movieclip (across movieclips, across tween frames), and it's quite a pain to have to name all the instance name. The best way to get around this is to create a class and associate it with the movieclip, and name it inside the movieclip itself: e.g.

class mc
{
private function onLoad() { this._name = "instance_name"; }
}

Much quicker and no need to mess with the FLA.