As a Spotify Premium subscriber, I think it is unacceptable that Spotify do not allow you to disable or throttle Spotify's upstream traffic, or even disclose how much bandwidth it is likely to use! I have an uncapped connection (common in the UK) but anyone on a capped line may be subjected to hefty fees without even realising it.
I have no issue with the idea of P2P - it saves Spotify money, which is hopefully transferred to me - and is essential for instant or near-instant playback of music.
This is also my number one complaint. I asked about it at their official support forum on getsatisfaction, but never got a response. That's silly, because Spotify sometimes saturated my upstream, in which case I turned it off. It has now become a habit to turn it off. Before that, most of the time, it was one when I wasn't even listening to music, so the bandwidth was completely free to them. If they would allow me to throttle it somewhat, they could use it all the time. Since they don't allow me to throttle it, I've turned to shutting the client down whenever I'm not using it.
Unacceptable? Well you are paying for it so clearly it is acceptable to you. They could have designed their service in another way but then you would have to 1) wait longer for songs to buffer before playing 2) pay more as their costs would be higher.
I have no issue with the idea of P2P - it saves Spotify money, which is hopefully transferred to me - and is essential for instant or near-instant playback of music.