You're asking a difficult question. Low-light visual perception has physical (which doesn't change from person-to-person as it's based upon inviolate laws of physics), physiological, and psychological factors. The last two factors are quite individual (e.g., genetics and emotions) and can even change for a particular individual due to diet, medication, smoking, age and perhaps some other factors i've failed to list.
Sure, these "microlights" don't put out a lot of light - just compare their light output in lux, lumens, or CP to their "big brothers". Furthermore, their battery source doesn't store a whole lot of energy, comparably speaking to even AAA batts (about 1/6 for a CR2032, IIRC and about 1/15 for a CR2016, IIRC). They can be pretty bright, especially the newest 2x-as-bright 5mm white LEDs (probably introduced in 2006 and found in a number of current headlamps). However, this limited battery capacity causes the output to dim markedly after only 30-60 minutes in many applications. Perhaps the best implementation, to date, of coin cell headlamps is the Princeton Tec Scout (you can ditch the headband and clip it to your cap/hat brim for hands-free operation). That's b/c it uses 4xCR2032 batts.
If you're young, with no visual problems you might be able to slowly negotiate, on an overcast night with a new moon, a non-technical trail using just a microlight. Many old geezers (like myself) will be more hard pressed to do so however. Snow, is always wonderful as it reflects a lot of light back to the eye, making it far easier to see at night.
Lastly, and maybe Rich Drehrer can help out here, the Mfr. specs about "throw" are, to me at least, a bit confusing. Some will be honest about how they measure duration (like down to 2lux output - you're probably not going to be seeing much with just 2lux output), but i can't recall seeing any info on how they measure "throw"/distance. Are they merely measuring a mere 2Lux at some distance to determine "throw"? If so this is totally unrealistic for actual use as only a small percentage of the light output, even in a dense forest, is going to be reflected back to the eye, and then it's traveling the same distance back, so should have only 1/4 the intensity due to the inverse square law of radiating energy. These tiny lights work fine in a dark attic or basement where there are a lot of nearby surfaces to reflect the light back, but out-of-doors, i find them to be woefully deficient for the trails that i hike on (especially when trees obscure any moonlight from reaching the trail). Personally, i've found a minimum of divide by three, or 1/3 of the Mfr claimed distance is more realistic, so when they say meters, i substitute feet, but this is my personal experience and limitations. I like a lot of light and like to see well so that i don't miss a bend or faded, low contrast blaze on unfamiliar trails.
IMHO, these microlights function fine as task/proximity lights. I could never negotiate most trails using one - a simple foot path, or out a couple hundred feet in the backyard from my house, 'ok', but not unfamiliar trails (especially if the foliage blocks any moonlight).
I think that you should have one with you as a backup - just like you said. At the very least, you can use it to enable you to see clearly to change your batts in your primary light source (if you're carrying extra batts). Plus for many in-camp activities, you can use it and save the batts in your primary light source for more demanding trail/terrain navigation.
Well, this post is too long already - 'nuff said. If i've failed to answer your questions, feel free to post back - i'll try to keep the next reply shorter.