At Disneyland, it's 10% off dining for the normal passport and 15% for the premium passport. It doesn't pay for itself at a rate of discounted spending of $100 per person per day until the 14th day for the cheaper pass (gives 10%), and the 27th day for the more expensive pass (gives 15%). I don't spend near that much in the parks, even with merchandise. The cheaper pass would save on tickets, if you're going for 10 days or more, and the more expensive pass if you're going for 15 days or more. The cheaper pass might be worth it with the discount for 8 days.

If you have a large group, I wonder if you can just purchase one single passport and have that person make all the purchases to get the discount.