Running sidelights in dreary conditions is proven to improve the visibility of your car.
Better to be the first one with your sidelights on than the last.
That's sidelights, not dipped beam.
Side lights as they are often refered to are actually parking lights. (I know we have another parking light function to put one side on only with the indicator stalk, but they are still only 5W bulbs at the front)
In the rain, especially when there is spray, are no good use at all for making your car more visible. In the mid 80's we were give dim-dip headlights. These were to prevent people from driving in conditions of poor visibility with only there parking lights on. For some reason in the mid 90's dim-dip headlamps went away, leaving people driving round with parking lights on once again.
If you have dim-dip still working, all well and good. But surely it's not too much effort to push the switch another click to put your headlights on properly. Next time it's raining have a look at cars driving with their parking lights on, and I'll guarantee that you will see the car before you see it's lights.