Note that below, I am saying that Dave is using the informal logical fallacy argument from authority. He thinks that a "succesful" musician isn't a political expert, and therefore, their opinion may be ignored. Meanwhile Tom Morello is saying the opposite, that it doesn't matter.

One basic problem with argument from authority, where the person making the argument is not the authority themselves, is that they are saying that they themselves cannot make the argument. So, how can they know whether to trust the authority that they trust?

It's not a bad default position to defer to an authority on a subject that you know little about, but that is not an argument that you should use to counter other people who have an argument.