Well, as a user, wouldn't it be nice if I could access 100 % of my battery for situations where I need as much life as possible? 20 % is fairly significant. It also cuts down on battery longevity, but plenty of people might not care at all.
For sure, a "extra battery" mode could make sense as the opposite of "low power" mode. Still, that smells like the sort of feature the average user would be superstitious of, like the obsessive app force-quitting I see folks do on the bus all the time.
