As others will have doubless said, dazzling lights are horrible, especially when they're mounted on a helmet as they tend to get pointed straight into people's faces. On the other hand, in my opinion at least, they do help visibility* and are great for seeing around corners when offroading.
The problem I think lies with the trend for brighter brighter brighter for cycling lights with little thought to anything else; when I was looking for a helmet light all I could find were really powerful ones with bright 'low' modes, or weak ones with no powerful modes and abysmal battery life. I ended up getting a Fenix E12 which is a little LED x1 AA 'penlight' (small torch), it has 8, 50 and 150 lumen modes; the 8 lumen is perfect for using on road to point at people - non dazzling, noticeable and about 30h of use, the 150 lights up an offroad trail well enough to see with. I mounted it with an Exposure mount and a thick rubber shim.
Short version: helmet lights on road should be on a constant low power mode and can be useful for being seen behind traffic.
*
-they can be seen over cars etc
-they can be used to point at cars on sideroads, pedestrians etc. to get someone's attention
-might help other people judge your distance when you have a second light on handlebars
