There is a big difference in having a headlamp for a couple of minutes on your forehead then put in on the shelf for shelf queening.
And there is the actual use of a headlamp for a long time.
When you actuality need a headlamp for a long time weight matters a lot to the point you don’t want to use unless constantly repeating in your head that your purchase was such a great deal.
Try to run wearing H30 and H04 ;))
It is safe to say we can agree 100grams is very little weight compared to 200grams on your forehead, again unless you want something of shelf queen headlamp which is fine, many flashlights are shelf queens anyway but the logic is pretty different.
While you can buy a 1.2kg flashlight for being a queen shelf, try buying a 1.2kg headlamp for being a shelf queen, you simply won’t.
You never seen people doing casual jogging with such headlamps but most have the plastic 30 lumen headlamp when running.
Depends on the application also of course, running requires very light weight on the forehead.
If the light is unsecured and clunking around, is that not just because you haven’t fitted the headstrap properly? 
This is true and worth reiterating. We can “get used to” these relatively small differences in weight. There’s nothing wrong with striving for a lighter solution because there are objective advantages to going lighter, but realistically there is not a hard weight limit which separates useable and not useable. As you say it depends on your use and priorities.
This last season I ran trails with a wowtac A2 (which weighs 56g without cell) and it worked fine. With a properly set up strap with top strap bouncing is not really an issue. Now, if you for some reason really don’t want a top strap then you would have to go a bit lighter I think. Personally I think I will stick with 18650 headlamps because I think it is a sweet spot in terms of weight and energy/power.
By the way I found this 18650 headlamp that weighs 43g without cell. I will be posting a review soon.
https://www.amazon.com/dp/B071ZVLNZL
Have you used a headlamp before? The straps are elastic and there will be movement unless you stretch them uncomfortably tight.
In my opinion, this shouldn’t happen with a well-designed headlamp, even an 18650 or 21700. I don’t have experience with the budget brands, but Petzl, Black Diamond, Zebralight H600’s & even Olight’s heavier 18650 right angles are secure enough for big, jerky climbing movements and unexpected 15+ ft whips into rock slabs.
I’m sure something like a Perun 2 or PL47 would be more comfortable and secure strapped to a helmet for that kind of movement. I probably wouldn’t use something that size for running, but I wear the H600 often without a helmet while climbing, and it’s perfectly comfortable. I cannot imagine the weight difference between a 21700 ZL & 18650 ZL being significant enough to make a difference, but that’s probably a testament to ZL’s design.
It all comes down to preference and use case, and mine doesn’t seem to be very representative of the BLF forum. For most people, using a smaller light and swapping batteries is probably ideal, like when camping, hiking or jogging. But in certain scenarios, a battery swap can be temporarily impossible or even dangerous. I know I’m being really specific to my own use, which probably seems niche here, but I’m sure it’s applicable for canyoneering, caving, SAR, etc.
I mentioned it in LoneOcean’s X1 thread, but I’d really like to see a Noctigon, FireFlies or Lumintop right angle with the new X1 driver and single emitter. Replicating ZL’s size, weight, durability and thermal efficiency would be unrealistic, but something even close with Anduril would be awesome.
Got a Perun, Mini and LD15R all with headbands. None of mine bounce and clunk around. I also have several other more industrial models that we get issued for work, which are primarily for helmet mounting but also come with pretty solid headbands.
Yes, if you actually reach your hand up and yank the light around like an arcade joystick, it will move… but most people don’t really do that.
We were working under the house running pipe. Wife ha 18350 and I had 18650. Wifes battery died and mine kept chugging along. 18650 is a minimum runtime for me. I wouldnt mind more but the 21700 is too heavy.