I always have at least a half-dozen NiMH AA batteries sitting charged in the drawer ready to go. When the mouse stops working, I grab a couple of charged batteries, slap them in, put the drained ones in the charger and then carry on with whatever I was doing. With a built-in battery, when the battery goes dead, I have to mess around with finding another mouse while this one charges.

(Yeah, the computer tells me when the mouse battery is getting low, but I do not have the discipline to remember to plug it in hours later when I'm done with the computer for the day.)