I had one of those! Also had (IIRC) a black plastic PowerBook G3 that had a big bay with a removable optical drive. You could pop it out and replace it with a second big battery. I frequently did that, which meant it would run for something like 4 hours instead of the usual 2 hours on a single battery! 😄
I think in a few years the batteries will have to be replaceable because of new laws, so maybe it will come back. I’m not sure what Apple is going to do..