onLoad as a dynamic event handler is typically used for loading in external data (via the LoadVars() or XML() objects).
onLoad as a dynamic event handler for a movieclip is pretty much pointless since the clip must exist to be targetted anyway. So basically…
myClip.onLoad = function(){
this._x = 500;
}
Is the same as saying…
myClip._x = 500
on the same frame. No need for the onLoad handler.