I think your right if you assume that the argument is a self contained object. But a lot of times an argument draws on someone's experiences. In these cases, it can be useful to know who the person is.

For example, I remember reading earlier comments about the negative impact of working all night long on a start up. If I had said this, people would (rightly) ignore me. But if someone who has a startup had said this, it should carry more weight.