Spotify Desktop's offline mode works technically but fails ergonomically . The lack of auto-detection and the manual 30-day re-authorization make it a power user's tool, not a mainstream convenience. If you need offline desktop listening, it gets the job done—but always remember to toggle offline mode before you lose internet.
Keep a small "Emergency Offline" playlist downloaded at all times. Before traveling, switch to offline mode while still online, then disconnect. This avoids the dreaded gray screen of death.