Discharging lithium batteries at a high C rate (C = Instantaneous Draw / Capacity) causes a different form of degraded life than storage at a high state of charge. And the major killer of lithium batteries is heat, which Tesla are known to manage well.

The biggest killer, though, is storage with the graphite electrode more than 50% lithiated - there is a gigantic step-up in capacity fade when batteries are stored with the State of Charge over around 90%, which Tesla are very smart to avoid.

http://jes.ecsdl.org/content/163/9/A1872.full