Besides the P2P, I think that with the native client they went after iTunes at that time. Their first versions of the client (not sure if current ones), scanned your hard drive to organize your music library, allowed you to purchase individual songs and also to sync your iPod. Now iTunes is not their main competitor, but Rdio and Pandora are, that's why I think the strategy shift.
Having a native client was the feature that caught my attention and made me try Spotify in the first place. I was tired of implementations that used to hang my web browser or consumed lot of memory. But that was back in the time. Their latest Mac OS client is very memory hungry, almost every time on my top 3 of applications consuming the most memory.