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Not having my front light suddenly run out of juice again during a fast descent in the pitch dark at 4am is worth quite a bit to me. Did not enjoy that.
I've had my dynamo light fail on a descent (a combination of water ingress and hitting a bump in the road), that's not fun either.
Having one light (whatever type it is) is a single point of failure.
For LEL/PBP length rides I had two 4xAA powered B&M Ixon lights. Most of the time having one on, on low, was good enough for night riding, sometimes I'd need to move it to the higher setting if I was in the sticks and moonlight wasn't good enough. On descents I'd generally have one light on low and one on high.
It also meant that if the batteries started to go in one (I could easily see the LED doing its red/green flashing) I'd move to the other light being the primary and then I could change the batteries at the next control.
For the qualifying rides (or just other Audaxes) I did prefer a dynamo setup but those rides (well, PBP at least) require a backup lighting system to be shown at the bike inspection. Some times you can just get away with wafting another light under the inspectors nose but being risk averse I just went for a dual light setup and it was waved through without question.
Greenbank-
I found my Supernova triple was fine for everything on TBPR although I did have a USB rechargeable spare - a little 200 lumen Moon light - as required by the regs. Never had to use it, but it's small and light so there's no real reason not to take it along just in case. On the Jura Brevet 600km ride I ended up using @Cycliste's spare front and rear lights after both of mine packed up, and I didn't have any spares. Won't make that mistake again.
Brommers
Not having my front light suddenly run out of juice again during a fast descent in the pitch dark at 4am is worth quite a bit to me. Did not enjoy that.