This is a big issue for me. Imagine another business trying to do this. What if your local supermarket (pre-self-checkout) required reservations to go shopping so they could adjust their staffing accordingly. That means if you prefer the short lines on workday mornings, if the market knows only 20-30 people will be shopping, they have only one cashier and one or two floor staff. That way they keep costs to a minimum. The line at the checkout will still remain 5 or 6 deep, and you still have to wait, even though the store and parking lot are mostly empty. Sure, the supermarket staffs accordingly for anticipated slower times, but if on a day they have an unusually low showing, at least those who show benefit from that. Many are probably even willing to pay more for that benefit. Just saying. However, if another business tried this you'd probably lean towards other choices in lieu of the company that is making life so complicated. I predict this could happen at Disney eventually perhaps increasing patronage at Universal, etc.
(Note: the above mentioned supermarket example includes completely fictional stats and is being used for example purposes only. I have no idea if any of it is realistic in general. Just making the point.
Last thing I want is to get into is a staffing debate with a supermarket expert.)