When using batteries in series the voltages add, when in parallel the current available is doubled. So having two batteries in parallel with no other electronics involved would cause them to balance their charge (one would charge the other) and then they should drain at a similar rate. My general idea was that one battery would be used before the other one, I'm pretty sure this is done with the power flow of one being used to control the use of the second through a transistor or two. But as far as reason and thinking... the reason why there are D cells and not just AAA is because sometimes you want a device to run longer or draw more power than a AAA battery can support.
If you are using a standard high end cellphone battery, the device is usable with just one, but if the person wants to buy 4 of them, and swap them out, quite literally more power to them. I believe that you would get the most use out of a battery in a slow discharge, because I know that standard NiCd batteries only yeald about 60% of their maximum mAh (miliamp hours) in a 3 min discharge (research I did for a friend who built a lightweight battle bot). But the ability to know what battery needs to be replaced when would be valuable. In fact using the segmented e-ink display from the lextor memory stick might be the perfect battery life indicator.
What I was trying to address with the suggestion was a way to make the device have variable power life, and the user could decide how much money to sink into it.
ok my brief stint into almost complete off-topicness is over!