Originally Posted by
cyccommute
Are you likely to drop parts of a flashlight? Probably not. But it can, and does, happen. There is no part of an external battery light that can be dropped.
Well, except for the old or new battery pack. (I'll assume you don't remove the light head to change the battery, which is probably a reasonable assumption.)
Unless you have both battery packs mounted simultaneously, in which case the appropriate comparison is somebody with two flashlights mounted simultaneously and therefore also has nothing to drop when switching lights.
If you're looking for advantages of one over the other, "fewer parts to drop" is a pretty minor one. (Unless it's a race and you can't afford to stop while you swap lights. Might be more important then.)