Helmet/Dash Cams are bought for one of 2 reasons..
1/ To record and show off ones hair raising exploits and bore ones friends and family!!

2/ To record evidence of an incident or in an accident.
In the second case they need to be good enough and mounted in such a way to be able to read the reg numbers..
I am not too keen on helmet mounts as they may become an issue if you fall on them..
I was nearly clobbered twice in 20 seconds on Saturday and my trusty Gopro recorder the reg plates quite clearly (play in HD).
Good Job I dont suffer from Road Rage....