At least from what a poster said, you can bank a ride later in the day of ots the only time available and still grab other ridrs before it after 90/120 minutes (TBD which one I guess).
But yeah, if they do not spread out the availability of will seem Genoe useless for late park goers. I agree. I want to pick my first ride st 930 for a 11 or 12 noon slot (with an hour window). And the first ride I'll want to be a prioritized one...like at HS with MMRR, to get it done first when we arrive. Genie + promises flexibility..let's see of ot lives up to it and caters to all guests preferences and arrival time.
Often people on arrival day head right to a park also and it's the afternoon already. WDW will lose put on Genie + sales to them if they have nothing they want available.