because they were lithium. packs more power for weight/size. Can maintain a higher output current in smaller package too.
so the 123 revolution came about because the light could be small, even though most arent at least the battery tube is
cause if your gonna pay 50+ for a light might just as well pay 50+ for batteries for it?
because Alkalines Sucked, for high continuous loads, even though there were many great rechargables to replace them.
I donno , you tell me , when they, here, almost totally abandoned the normal consumer batteries for all the great lights , i didnt know what the purpose even was , they all crasy i tell ya lovecpf
the 18650s are a power house of capacity and ability to handle load, then the fear of the battery EXPLODING , caused people to waver around wondering what to really do (remember the FEW laptop fires and that panic).
the 18650 has about as much power as 3 ni-mh AAs , yet is in a very pocketable size and is lighter, until they put a Flashlight head on the light the size of a elephant
and enough metal to make a club out of it.
many 2x123 lights that will use both the 2x123 AND the 18650, run the 18650 at lower power, just because the curcuit in them was desinged for more voltage, there ARE lights that stay fully regulated with both, but many were (sorta) direct drive with a 18650, giving the advantages of very very long dwindling runtime.
of those types of lights (using both) you could still use BOTH and get the advantages of both depending on the need at the time.
and dont forget there are 123s AND rechargeble versions of the 123 ALSO, so not only is there the rechargable 18650 li-ion , and now safer versions, but there is also rechargable li-ion 123s of many different types, each with some differences too.
if your going to use a 14.4v 4 X li-ion pack for a bike light, then a hobby balancing charger is going to come in VERY usefull. because you are going to want to either charge as seperate pieces, or charge using a Balance charger which will treat each cell in the pack to a proper charge.
Sooo
a hobby balance charger will usually ALSO charge single piece li-ion cells, giving you a tool that will do both.
And
once you had one of them things, then you will certannly want to get other li-ion batteries, or mabey even li-fe-po one of the safer versions of them, so you would want to get a charger that can do both.
at any rate if you step off into the land of Flashaholic and use lithium primaries (throwaway), then you should heed the DON'T MIX rules , dont mix brands type age and discharge levels in a device when using series batteries.
and if you go rechargable Li-Ion , make sure you know some of the specs and care needed to use them properly.
and if you get li-ion make sure you know what your getting in type, so you can know how to deal with it. learn what protected is.
THEN
everything will be fine and it will be a great battery type, and kick butt and be really lite , rechargable, and have lotsa runtime for you.
if you dont want to Think, read specs, understand, or deal with it, then get some of the great AA type of lights, and you can put AAlithium cells in (most) of those TOO, and get lightweight long runtime, and still have the power of lithium in your hands. still would want to check to see if the light will handle lithium cells.
at any rate for hiking, the LED is one of the biggest helps because you dont haveta haul so much power around, because it doesnt USE as much power, then on the other hand, you want to be carefull how BLUE your leds are , because there isnt anything blue outdoors
just from one Hiker to another.