Vote this issue up if you have an account!
https://community.spotify.com/t5/Ongoing-Issues/Major-I-O-wr...
"We've seen some questions in our Community around the amount of written data using the Spotify client on desktop. These have been reviewed and any potential concerns have now been addressed in version 1.0.42, currently rolling out to all users."
So... sounds like it's fixed? If so, good job on making a fuss, everybody!