Personally I put Spotify at the same tier as piracy. I mean that not in terms of morals or ethics but in regards to the viability of the industry. If you want to copy all music ever recorded that's fine, go for it. We're almost all guilty to some extent. If however you'd like to see new music created in the future then you owe it to yourself to compensate the artists you enjoy.
The most dangerous thing about Spotify is that people genuinely believe listening to a few ads an hour or paying a mere $10/month is enough to support the industry they love.
[1] http://www.digitalmusicnews.com/permalink/2012/120730infamou... [2] http://www.businessinsider.com/spotify-revenue-2012-10