The problem with that is that you really want the butter soft, not actually melted. There's a difference in baking between soft melted butter and liquid melted butter; it separates out and loses all consistency and doesn't do the right thing in some recipes. And when you microwave, there's an incredibly fine line between soft and melted, and it's one you can't reverse. So sitting out is safer if you want "proper" consistency in your cookies. :)