I've seen the following argument a thousand times: "Well the marginal cost of a copy of [some digital good] is 0 so it isn't stealing!" That argument is actually completly irrelevant. I took a train the other night, and there were only 2 people in my whole car. The previous week it was packed. Both times I had to buy a ticket at the same price. If you don't see what that has to do with pirating music (or software, or…
But even when I do everything myself, I'm using high-end equipment and software that I've paid thousands of dollars for. It's not free of charge for me to be able to record music. And even if I were to deem those to be irrelevant sunk costs, to assume that my recordings ought to be free of charge just because they easily can be copied suggests that my time and expertise is worthless, if nothing else.
[Edit: of course, I'm just some random guy on the internet, and certainly not nor attempting to be a full-time musician. I'm presently content to make money building software, and any income that results from my music is just a bonus. Full-time musicians surely feel even more strongly about such matters.]