I don't actually know why I stock up on crowns. I still have 21k crowns from the last crown sale (Though I have to admit that my ESO+ renewed in that time, so I've spent a total of 9k crowns since the last crown sale).
Zero. I've only bought crowns during one sale, and that was the ~3000 pack. Still haven't spent most of them, and that was back last year sometime. I also had some crowns left from ESO+ months. In the last year, I've bought 2-3 cheap costumes, and the Thief/DB DLC when they were 500.
(Yes, the 5500 pack is the "better value", but I didn't need that many crowns nor did I wish to spend that much on the game.)
I've only played long enough to have seen one sale, but for that one the discounts were based on the quantity you bought; e.g. the 3k pack was like 10% off but the 21k pack was close to 50% off.