If artists see streaming only as a way to promote themselves, because they don't get an appreciable amount of money from it, why should I pay for it?
If by not paying for music, you mean pirate, that is too much effort for me. I can give Rdio money and all the music is just there, ready for me to click play.
If by not paying for music, you mean the artist give it away for free on their website. I have to put effort into finding all the artist that I would care about and then finding their websites, and then try to figure out how to get the music on their website. Again, that is a lot of effort. I am paying to not expend that effort.
All that aside, I am not trying to say music services are better, just how I justify my money on them.