Kate Knibbs, I think you’re missing the point that Spotify absolutely depends on listening sessions being ‘un-private’ in order to track what you listen to and make recommendations to you. Enabling private session means they do not track your tastes at all, period. They will have no idea what you like!
Spotify arguably has the best recommendations (see their ‘Daily Mixes’) in the business. If you make every listening session private, they will no longer have the ability to help you discover meaningful new music. Kinda defeats the whole purpose of using Spotify, doesn’t it?
Just get used to turning on Private Session when you are indulging a guilty pleasure. Spotify says “The Private Session will end when you restart Spotify or after a long period of inactivity.”
I’m assuming you care about getting great music recommendations from Spotify, which they have the best reputation for. If not, switch to one of their competitors for lousy curated recommendations or just use Pandora if you don’t care at all.