It still is relevant if you have a team (team diving).
There has been a time or two when I have had to use my backup, surface, fix my light (or swap it out), and my buddy has handed me a fresh set of batteries from his save-a-dive kit. One set of batteries for 2 divers. Real convenient. Just like that, we're ready for another dive with minimal hassle.
I carry a spare bulb/reflector in my save-a-dive kit. It will fit a scout style (rat, whatever) light because the body diameter fits c-cells.
If you want to dive a cr123 powered light, thats fine. But for team diving in remote locations that require a fair bit of effort to plan, get to, and dive, simplicity and interchangeability are very important.