The primary benefit for me from rollover nights is the double counting of lifetime nights. I have 1,000 lifetime nights, and about 100 of these are from the rollover nights.
Another benefit is that rollover makes it easier for people to keep platinum for several years in a row, which makes them eligible for a grace extension of platinum status. Marriott will manually look at each platinum account that has been platinum for 3+ years to decide whether to extend platinum status even if the person does not have the necessary nights.