One other concern: What's the quiescent drain of this boost regulator? For some very low-drain devices, or applications where the device is left switched off for long periods, will the quiescent drain kill the battery, or at least cause it to die faster than it would have died without the battery sleeve?
Sometimes the battery management really should be in the device, not in the battery sleeve.
Well, that's it isn't it? If using boost converters on the regulator side INSIDE the device was a great advantage, manufacturers would jump at it to get extended battery life. I imagine that very low power devices could benefit, but not anything that has to continuously transmit signals or draw any real current.
Why would they? They don't have to buy your batteries. It
IS a great advantage, the problem is the device maker doesn't care, it's the consumer that cares, but only after they've already bought the device. It's the classic problem of the device maker being completely separated from the maintenance costs of the device.
Building construction companies don't have to pay your electricity bills in the summer, so they don't care about how much/little insulation is used unless you go out of your way to request it. Instead they cut costs to minimize their bid, and you're the one that has to pay in the end if you didn't have the foresight to specify double/triple paned windows, extra insulation, etc.
Some devices could be advertised that they have minimal power consumption, but some can't. Nobody is going to buy a TV because the box says the remote control batteries last twice as long as the competition. Instead they just minimize cost and you're left swapping batteries every month, or buying your own universal remote.