I think the "next available" is what makes that acceptable for the most part. If you want multiple rides with high demand, you'll have to use your earlier selections to ride them later. That ultimately gets you fewer rides total as you can't use any skips in the early time. They initially were letting people book a new one as soon as they used "any" previous LL, plus the 2 hour window. Thankfully they closed that loophole where you could pick a new pass and then immediately use an earlier one.
If we start letting people choose which time their passes are for like ILL, there could be different issues with passes than we have now. So I think the stacking aspect at least lets you hold onto something good without forcing you into just one pass if you want to ride SDD that day and got one for 7pm.