I find that Quora answer interesting. I get the argument that Uber is building for the future, where cars-as-appliances just hang out and pick you up when you call for them. But if I know I'm going to need a car at a particular time, why wouldn't my Uber of the future want to know that it needs to adjust the random distribution of cars-for-hire, or logistically coordinate more rides to end up near me, for that time?

If Uber knows it needs a car at 3pm, it can push work availability notices to drivers near the pick-up location who may not at the moment be working. It would be a benefit to the driver to be able to have the guaranteed fare, and if the driver is late Uber can just give it to a different driver. Pre-booked cars can be priced differently for the driver or passenger to ensure the opportunities are attractive enough for the slight inconvenience of having to drive a bit more to pick up the passenger. And it's more convenient for the passenger to know a car will be there.

Pricing can still do its surge thing, or they could require a pre-booking fee or something.

I don't see how ^ would require a fundamental reengineering of the system.

(And, as a round-about way, I agree allowing pre-booking would be a reasonable requirement, if it's indeed an existing statue.)