The problem is that the beams from visible light are influenced by any other lights in your scene that intersect that light beam. You can see this for yourself by making a plain volume visible infinite light with no fallouff in an otherwise empty scene. Now, change the sun color, and watch the light beam change color, too. Make the sun color dark, and watch the beam disappear. Create a spherical light, take it into the Light Lab, and set the preview to "Render in Scene" so you can see what's going on without leaving the lab. Change the color of the light, and watch your visible light change, as well. Turn up the intensity, and watch your light beam get brighter. Turn it down, and watch the beam fade. It's annoying as all get out, but there it is. In order to make the Visible lights work properly, the other lights in the scene must be at least as bright as you want that light beam, or they must not intersect that beam. For a lot of the things that you mention, it's just easier to use an object instead. Cylinders with 100% transparency, and ambient color and ambience to taste make great laser beams, for instance. Cones with a good texture and blend transparency to give them ragged edges work well for muzzle flashes, etc. If you want more specifics, let me know.