I was thinking about this last night... possibly in my sleep....
If a device NEEDS, 1.1volts per cell, or 2.2volts total for something using two AA cells in series, then logically, if there was energy below the 1.1volt per cell cutoff, then wouldn't the manufacturer design the device to run off three AA (or AAA) cells in series, so the 2.2volt cut off occurs at 0.75volts per cell...
But then I also thought, it's easy enough to see how much longer you can run from depleted cells, by taking a device that's meant to run off two, and wiring three in series, once they're already depleted of course (or in a device you don't care about)
No boost circuit required!