Most light aircraft - Chieftains, Cessnas, Barons and so on, use full power for take off. In the POH for these aircraft there may or may not be a table of take of speeds for actual weight. I have never seen a manufacturer's recommendation to use the speeds for the performance limiting weight instead of the actual weight.
If therefore for some reason an airline decides to use full power for every take off in a 737 regardless whether or not the aircraft is light or heavy, then it would seem logical to use V speeds for the actual BRW just like a light piston twin.