When an o-ring is at least partially visible from outside if it is luminous it will help you finding the flashlight in the dark. If on the contrary the o-ring is internal I agree with you that it is totally useless.
I just like them cause they look cool.
In case the o-ring is not visible from the outside I am afraid I fail to see its usefulness. You probably refer to the ones behind the lens which are in any case at least partially visible from outside.
Actually, I've replaced stock o-rings (not visible) with Sigman's glo-rings. The silicone rubber they're made out of is of high quality and improved the operation and waterproofness of a couple of lights I had. It's all a matter of finding the right size to fit your purposes.In case the o-ring is not visible from the outside I am afraid I fail to see its usefulness.
Actually, I've replaced stock o-rings (not visible) with Sigman's glo-rings. The silicone rubber they're made out of is of high quality and improved the operation and waterproofness of a couple of lights I had. It's all a matter of finding the right size to fit your purposes.