I give $10 to Rdio every month instead of Spotify because Rdio has a better app. Rdio's catalog isn't quite as extensive as Spotify's, but I can deal — I've got friends on both, so the social experience is a wash. There are good reasons to use both!
There's no good reason use any of the others: MOG, Rhapsody, Napster and Slacker all lost relevance as soon as Spotify took off. That's why Napster sold to Rhapsody, the oldest and least interesting streaming site, and why MOG, which used to be the cool, "curated" music streaming service, sold to Beats today.
MOG will probably end up getting bundled with overpriced headphones and God knows what's going to happen to poor Napster. Selling is death for these companies, but they've all been terminally ill for a while. Their catalogs aren't any better than Spotify or Rdio's and neither are their apps. Without a meaningful number of users their social networking features are totally pointless, and none of them can afford to undercut the others on price. It's not their fault, really: Spotify came to the US with way more buzz than it deserved and Rdio was an early darling of tech writers and Silicon Valley types, thanks in part to the company's generously long free "trial" subscriptions. Plus, the whole unlimited music model is predicated on the destruction of wealth, which means making money is hard even if you're winning.
But it also doesn't matter. As of today, every service that isn't Spotify or Rdio is on deathwatch. And maybe we should keep an eye on Rdio too.