I've never found a light to be useful in low visibility, if anything it makes the situation worse by causing reflections off the haze, no different than using high beams in a fog.
If you're referring to using the light to make it easier for your buddy to see you that's something else. But I wouldn't dive in extremely low visibility conditions with a buddy that are so poor you won't see them unless you catch a glimpse of their dive light. Odds are high that there will be a separation and if both are not self reliant, so equipped, and comfortable diving alone, and find themselves in poor vis which can make orientation difficult, that's a big problem.