Years ago, back in 2001, I had a nice Dell laptop with a modular DVDRW drive. However, you could hot swap out the optical drive for a second battery pack. I pretty much ran with this second battery pack in all the time, and it was awesome. It added an extra 60% or so of extra battery time to the laptop and I could go a real-world six to eight hours of use before the power ran out.
My new MacBook pro has a DVDRW drive in it and it's just complete wasted space. The battery life for this MacBook Pro is already pretty good, but it would be very awesome if I could put a modular battery in there. FYI, I have one of the first generation of unibody MacBook Pros, so I can very easily get to the battery and hard drive. I loath the fact that they un-did this feature of the MBP in later models. Jerks!