DRLs use the high beams at a reduced power setting, not the low beams, because the high beams are aimed at the incoming drivers' eyes. Euro-pattern beams with sharp low-beam cutoffs specifically, are designed to not glare the oncoming driver, and hence are not good DRLs.
i use my foglamps as DRLs, since my car is a 1989 190e. The advantage of using them vs. high- or low-beams is that the beam is spread over a wide angle, allowing one to be seen by drivers over that angle, including those you are about to overtake on the highway.
here in canada where all cars after a certain year have DRLs, a car without DRL will be lost in the driver's field of vision among all the other DRL-equipped cars, especially in inclement weather.
with my foglamp on, the rear lights are also on, enabling other drivers to see me better from the rear in inclement weather. Most cars with DRLs do not have their rear lamps on while the DRLs are on.
what i cannot understand however, is why they removed the rear foglamp, when it is most useful for being seen in heavy rain and thick fog, especially on the highway.