Water parks are not open late, they close at 5 through most of the winter and stay open until at most 8 during the rest of the year.
With no direct busing, it’s also time consuming to get there. So, you’d have to arrive at your hotel relatively early in the day to make use of this perk.
I’d say it’s fairly useless to a good portion of visitors - between colder winter weather and afternoon arrivals.
My guess, WDW has an eye to the opening of
Epic Universe and this was a cheap perk they could throw out that there looks good (free park entry!) that isnt particularly useful to a large number of visitors.