My sister called yesterday, and we were trying to think of a way that an all-FP system might work for us. Other than the probably-mathematically-impossible legacy-style flexibility that I mentioned way upthread, I think it would go a long way if we could hang onto more than one FP at a time. The logistics of the thing might be impossible, but it sure would be nice.
Granted, being able to experience two attractions in an hour would really defeat WDW's purpose, of making us shop and eat more. But from my family's perspective, the idea of having to schedule all our rides, one per hour, really doesn't work. We're typically morning park tourers; we leave after lunch, spend the afternoon poolside, and go to drinks and a nice dinner in the evening (perhaps in a park for fireworks, but by the end of the day we're less interested in running around for rides anyhow). Which makes us already WDW's ideal guests, right (we stay Deluxe, too!)? But if we can only ride three rides in a morning...well, then, it wouldn't really be worth it for us to be in the parks at all.
I suppose they could tier it, give you the choice of one e-ticket in an hour, or 2 d-tickets, or 3 Dumbos, or something. That would just make an already-complicated beast even more complicated. But, really, if you've chosen Dumbo for an FP, that leaves a whole lotta minutes left in an hour to fill if you can't ride anything else in the meantime!