That's not actually true. The on-site guest access at USIOA is *unlimited*; you can bypass the line on any ride at either park as many times as you like for your entire stay; we rode Spiderman 8X in a row once. The extra-cost "Express Pass Package" that you can buy at each park in the morning gives you one-time Express access to each ride in that park at any time that day.
Otherwise, you're right. The free version of the Express Passes go very fast. We were there in at the slowest times of year, and all of the machines for the free Express Passes were hooded by 11 am in both parks; I can't imagine how fast they would go at peak season.
WDW can't give onsite guests unlimited head-of-the-line access b/c WDW just has too many onsite rooms. I think that USIOA's onsite guest capacity is around 6K even if every room is full. WDW's onsite guest capacity is around 130K. One thing I do think would be nice for WDW to do for onsite guests, though, is to allow what is sometimes done at
Disneyland; give out one "use-it-anywhere" FP per *stay* per guest. A FP like this can be used at any time on any ride, but once it is used, it is gone.