Dave, I agree the batteries are very light. I'd like the option to have a larger battery for longer run time (not that the run time on the current batteries aren't very good). And, since the battery is the bulkiest part of the drill I thought it might be nice to have an even smaller battery for the times when run time isn't the biggest concern.
As a cabinet installer, I'd love to see smaller batteries. Run time isn't so important to me, as I'm always on powered sites, but I would love to shave an inch or two off the overall drill size (or even some fractions of an inch) for those small boxes that just won't accept the t-15.