Only 20% of users have some kind of paid Spotify account, either unlimited or premium, so that's a rate of between 0.7 and 1.5 cents per stream. Even for that 20%, that's significantly less than the approximately 9 cents per track that an artist would make from downloads on iTunes or AmazonMP3. Not glorifying iTunes or Amazon in the slightest, but Spotify really isn't an incredibly legitimate source of compensation for an artist. If I have a premium account, I'd have to go listen to a song six times to equal the pittance Amazon throws an artist's way. If I don't have any kind of paid account, like 80% of Spotify users, I have to go listen to it SEVENTY-FIVE times to equal that pittance. You're telling me most Americans have the dedication and attention span to return to a track seventy-five times in a relatively short time span? Nope, don't think so.
You seemed, however, in your original post to be alluding to a more interesting proposition, which is that people go and stream things on Spotify for free that they wouldn't pay to download. I definitely think this is true, but there's no way to track that, and I am highly skeptical that that factor alone makes up for the egregious inconsistency between what Spotify pays artists in comparison to other outlets.
Edit: Did some independent research and number-crunching, and the most recent figure for what percentage of Spotify users have premium (not sure about unlimited) is about 11.5%, just FYI.
So because it doesn't pay as well as itunes, a one time purchase, we shouldn't use it? It's a hell of a lot better than pirating and how likely are you to but something that you will only listen to less than 6-75 times throughout the life of owning it? The upfront payment is less, the long term payout is more and the fact that you are making something off of a song that would otherwise just be pirated and you'd see nothing is clearly better.