Largely because I've yet to use a streaming service that handles intermittent connectivity well enough
If you have an Android phone: Google Play All Access.
You can offline content to your phone as if you own the music, but you get full access to their complete library. It's pretty brilliant, IMO.
Edit: though that doesn't help you if you're looking for a radio-like service... I tend to listen to albums at a time, and discover artists through third party services (e.g., Rate Your Music) that then drive me to sample stuff on Play. So it works well for me.