You are right about alkalines, they are definitely marked as 1.5V nominal. It is the reasoning behind the warnings on Gameboy manuals to not use rechargeable batteries (1.2V), but they worked just fine nonetheless.
The low battery indicator doesn't work particularly well with recharchables.
With a standard non rechargeable alkaline the voltage drops gradually as the battery discharges, so the battery light coming on at a certain voltage gives the user enough time to save their game and replace the batteries.
Rechargeable AA batteries have a much flatter voltage curve - they basically give a stable 1.2V until they run out of juice and then drop rapidly.
