I doubt they do much. If you're already in the headlights of a car that's pulling out of a side street, it's probably too late to prevent anything. You should have good lights so that you're noticed in advance. And those things get covered in dirt anyway and are a pain to keep clean. However, I see no harm in having them and theoretically they may do some good. So you can get them, of course, but don't rely on them to make you visible. Active lighting is a lot more important for cross-traffic visibility.