Cell packs can become slightly unbalanced or by user error you can put 2 dead batteries with 2 good batteries, but then they will discharge into each other anyway and really cut the lifespan

One interpretation could be that each battery powers a separate LED, and thus they aren't connected and if a battery goes dead that LED just goes unpowered.

So aside from the mechanism, it also potentially means redundancy and higher cost (more parts). I'd say maybe the LEDs ran by AAA might last longer (same LED configured to run lower so the battery lasts longer), but LEDs themselves probably aren't the thing burning out in most cases.

As a bonus it'd mean that it'd be easier to know when your batteries are low (especially if high-and-low placements meant batteries go dead one-by-one rather than all-at-once), a problem for me as I keep using my flashlight until it's super dim (somewhat because it's fine, somewhat because I don't notice). Though I guess that could be an issue too, I know the battery charger I have charges batteries in pairs.

That last part said, it might be better to use the AAA batteries as backup (or maybe extra light in a high-power mode) instead.