Lava wrote:potholer wrote: having a spontaneous light loss on a climb is also unlikely, assuming someone is using a proper light without a tendency to peril-sensitive failure.
Having come of age in the in the bulb era of caving, I've experienced more spontaneous bulb burnouts than I can possibly remember, and during all kinds of situations - climbing, on rope, hopping boulders, etc. So you'd be hard pressed to persuade me to take that second light off of my helmet. Aside from bulbs, I've also had electronics fail and batteries die. I'd agree that modern lights are an order of magnitude more reliable, but from reading these and other forums plus talking to other cavers, I know of failures in Sten lights, Zebralights, Sparks, El Speleos, and Scurions. So I think it's far more likely to happen than you are implying.
I'm not making recommendations for others, just talking about my experiences and my choices based on what I know of my system.
Even in the old days I had hardly any electric light failures, but I normally used a non-halogen bulb in my Laser headset to back up my carbide, and those bulbs seemed pretty durable, not that I tended to use the electric except in wet/windy places since the carbide was reliable if properly maintained. I did have a cheap 2*AA flashlight and then a mini Maglite as third lights, but I'm struggling to remember using either of them except as loan lights to other people. I did sidemount the Maglite for a while, but only because if I carried it hanging in my personal gear bag the bulb kept falling out of the holder.
Also, I'm in an unusually good position to know how reliable my main light is since I made it, a decent number of other people I know use one, and partial or complete failures would very likely be brought to my attention. It was built with reliability very much in mind with worthwhile redundancy within a beam driver, and completely separate drivers for the two beams so the business end is effectively two lights already. The weakest point is the battery connector on the end of the (heavy duty) power cable into which various battery packs or AA holders plug. Chances are that if that did fail suddenly it would be at battery change, not when in use when it's under no mechanical stress. I'm only aware of a failure there happening once, and that was bodgeable in the field to a usable state.
However, even if that wasn't the case, if I expected that failure of an off-the-shelf light was going to be as rare as even every 2 or 3 years, let alone longer, that'd still make me reluctant to helmet-mount a backup
if the only practical mounting places would expose it to meaningful mechanical abuse.
In such a situation, if I
could sensibly front-mount something like my ZL backup, I'd consider it, though I do like solid mounting rather than things with headstraps, and solid mounting the ZL would probably make it at least more awkward for me to use it for other things, including as a helmet-off light while camping underground.
Certainly, for someone who has no idea how reliable their main is, there's an argument for at least starting off with a backup instantly ready on the helmet, and these days, depending on the main light's characteristics there could be an argument for having a helmet-mounted backup as a possible complement to the main, providing a useful beamshape and/or output which the main light lacks.