My own experience suggests that a very important aspect of a helmet mounted light for bicycling is "spotlight" and "throw" performance. But this perspective about helmet lighting assumes you already have adequate bicycle mounted road lighting.

In other words, if you use only a helmet light - it needs to be a great "all around" bicycle light . But if you already have a good bike-mounted light, then you can benefit even more by having helmet light that has great "spot" and "throw".

My experience is that a spotlight on your head helps you search out dangers along the road and offers a additional visibility/recognition when aimed at inattentive motorists. But again, this is only a good idea if you already have a good "flood" light on your bike.