I ride with a helmet light and in the past rode through some of the lower income, high pedestrian areas of Atlanta. The light effectively stops the (potential) interview phase of criminal interactions. Someone approaching with (potentially) ill intent will usually stop and start talking/gesticulating where they are once I shine the light in their face. This allows me to exit the situation or attempt to dissuade them verbally if I cannot leave. A helmet light needs to be a part of a larger strategy or de-escalation, movement, and/or a self-defense continuum.
However, if someone is coming to criminally interact with you (assault/battery/robbery/etc.) without any soft approach, a helmet light will not dissuade them. The same behavior is seen in dogs, an aggressive dog coming to play/herd will be dissuaded by a bright flashlight in their eyes. A dog coming to attack will only avoid the beam and will maneuver as necessary to get past it to bite you.