The incentive would be that if one downloads between 0:00-6:00, the shaper should not count this data when it is deciding who to shape first.
Let's give an example.
2 customers with 4mb lines:
Customer 1 only
downloads between 0:00-6:00 (8-9GB) for a month (x30 days) and
totals approx 300GB.
Customer 2 only
downloads between 9:00-17:00 but
only pulls 150GB for example.
Even though customer 2 is putting more strain on the network when it really counts than customer 1, customer 1 will be shaped first because of his higher monthly total, and I think this is total BS!
With IS and their star rating, for example, if one downloads during these times, this doesn't affect the rating, therefore incentivising people to use the off peak times, and rewarded with lighter shaping during the peak times for important stuff.
Moral of the story, follow IS example.
Use it don't use it