Folks, you can own multiple rechargeable batteries and they're hot swappable, at least the 18650/350. No different than a powertool or any other battery tool.
Well, except the design of the Surefire and OLights...which is what makes them actual non-starters (I feel like that was said before, by someone in this thread...).
I run rechargeable AAA NIMHs in my daily carried light and 18350s in my long-gun lights. I change them once a month, because of preventative maintenance. Not because I've ever run a set down, ever, but because Prepared. Right? Right. Throw them in the charger when they come out and store them afterwards. I admit, I'm not a big power user. Some folks are going to run their lights 1000x more than I do and need to carry a few spares on their person.
How many folks here have had a light just drop dead on them before? I have, multiple times, I carry spare batteries like I do a spare magazine. (Well actually, no, I usually carry spare batteries in my vehicle or bag). If it's a rechargeable or single-use battery it doesn't matter, they swap the same.
Honestly, if folks want to stay with a CR123 light - that's fine. But the excuses against rechargeable batteries here are really quite silly, most of them don't make sense. The exceptions are institutional use or if you're running a bunch of stuff that use the same batteries, like @theJanitor - that's the cost of business.
Personally, I don't have an NODs, optics, or IR illuminators, etc that use CR123s (I don't have NODs or IR Illuminators, period). In fact, the only thing I have running CR123s are flashlights and a pistol light. Eventually (by end of the year), I'll have zero devices running CR123s, because all my optics will be running CR2032s and all my lights will be 18350, AA, or AAA powered lights (because our life ecosystem says, I'm never getting rid of AA or AAA batteries).
If you have a decade's worth of CR123s on hand, though, by all means, buy a CR123 powered light. For folks who want a CR123 light - make sure you examine Elzetta's offerings in addition to Malkoff and Surefire. Remember, Elzetta and Malkoff have been around a long time and have a lot of dedicated users.
For folks who want a Modlite handheld with two modes. I say check out the Cloud Defensive MCH. The reason CD has a two mode head available is because users requested that feature. No strobe, no programming, just a HIGH and a LOW mode. It's basically impossible to do admin and 'tactical' tasks with a single BRIGHT mode light. Since handhelds are multi-task lights, that's why Cloud was convinced to do this. Their weapon lights are pretty much all single mode BRIGHT lights. You can now get the two-mode format on the REIN 2.0 (since they use the same heads as the MCH), but I don't see any compelling reason for this and would advise anyone to go just single-mode.