Originally Posted by
dougmc
This isn't really a fair comparison.
Flashlight steps :
Remove flashlight from holder
Remove back of flashlight
Shake battery out
Put new battery in
Put back back on
Put back in holder
Wired bike light steps :
Unplug light from battery pack
Remove battery pack from bike
Put new battery pack on bike
* note that the previous two steps may be non-trivial.
Plug light back into pack
Now, if you have your spare battery pack mounted somewhere so you really can just plug it in, that's fine -- but really, if you can do this, you can just have a second flashlight mounted too ready to go with even less effort and the same space taken.
This does get more complicated if your light uses multiple batteries (be it wired or flashlight) but the ones that most of us use use a single battery or single battery pack.
Personally, I don't just carry spare batteries -- I have a spare light or two. You never know what'll happen. Ideally, I've got at least two lights mounted on the bike ready to be turned on at any time.
Removing the flashlight from holder isn't a real problem. Removing the back of the flashlight is where things can start to go wrong. It's a small threaded part that is usually black. You now have two parts that you have to keep track of. You shake out the battery. You now have 3 parts that you have to keep track of. You get out your new battery which gives you 4 parts to keep track of. Damn! You fumble the end cap. Now you only have 3 parts to keep track of but one of them is somewhat hypercritical.
You can't really do the same thing with an external battery pack. If you drop the pack, it's larger and less likely to roll off into another dimension.
Sure you can carry multiple lights and only run one. Personally, if I'm going to carry an extra light, I'm going to make it work for it's ride. I also carry enough batteries to get me through the commute in and the commute home. If I need, I can actually run 2 battery packs in parallel to extend my range. You can't do that with a flashlight. It only carries one battery and has to be disassembled to replace that battery.