Today I happened upon a cyclist riding a recumbent bicycle. I almost couldn't believe how well he blended into the surroundings ( as I almost didn't even see him ). He was going the opposite way and when he passed I looked in my mirrors and noticed a little dinky (single?) red led light on back of his helmet. It was so dim that I almost didn't see it although I could see the rider without too much problem ( early evening and still a bit light outside ). I couldn't help but think to myself, "Geez, why even bother"? By the time you see the light you are almost right on the rider.
This is when it dawned on me, " The purpose of a decent lamp is to get the rider seen by the motorists BEFORE what would normally be considered "ambient visual contact range" (without a lamp ). In complete darkness this is pretty easy to do and doesn't require a really bright light. Still, having a brighter light will generally be more noticeable ( although as noglider has pointed out there is a point of diminishing returns ). This is even more important when other bright lamps are in the same area and are competing for attention.
One of the reasons I like "flashing ( blinkies ) lights" is because I've found over the years that when ever I see a cyclist using one, "It's the BLINKIE" I see first THEN the cyclist. This is not always true when I see a cyclist using a lamp with a steady output. This is why I personally use both. Steady light for seeing where I'm looking and the blinkies ( front and rear ) for being seen.
Personally I don't assign mystical powers to bike lights. They have their effectual limitations. I'm not going to argue over "What is bright enough or what is too bright". To each their own. I've had people tell me they could see my ( rear ) lights a mile down the road. They also said they didn't think it was a bike but were fascinated when they got close enough to see it actually was a cyclist. That made my day. Now if you ride areas with shorter lines of sight a brighter light is probably not so important
Parting thoughts: Having a brighter lamp isn't going to increase your chances of getting killed. On the other hand choosing NOT to use a bright lamp isn't going to increase your chances of staying alive. Find that "happy medium" that gives you better road presence. Your choice, now let's all spin that cylinder and hope for the best.