I don't think they make cars any easier to see in broad daylight than they were already were.
The experts seem to disagree:
http://www.swov.eu/rapport/R-97-36.PDF
Which is why we all have DRL's now.
A ton of hypothesis, theories and assumptions in that report - pretty light on actual facts. One example of the inconclusive stats it uses shows adverse DRL effect on visibility/instant recognisability for motorcycles,causing more motorcycle related accidents upon the introduction of DRLs in Denmark, but dismisses a 16% rise in these accidents as insignificant, yet excludes them from the data they use to try to demonstrate a reduction in accidents with DRL introduction. There are a lot of shaped and cherry picked stats in there to try and prove the point. Lots of waffle in there to try and mask preferential use of data that supports their hypotheses and exclusion of that data which does not. Some parts of that report sound like they came straight from the mouth of a political spin doctor.
It would be difficult to put a number on the effect of DRL in reducing accidents when so many other factors come into play - better brakes on newer cars, tech like ACC and lane assistance, speed cameras everywhere making people more aware of their speed and the fact that many cars on the UK roads don't have DRLs are just a few. On a poorly lit day you should certainly have your lights on, but on a sunny day? Definitely not convinced.
How much awareness of the car behind prevents accidents? It's awareness of the car in front or alongside you that prevents most accidents (and you can't see the DRLs on the front of the car from there), as the car in front can do very little about the car behind running into them.
Most accidents happen as a result of sideswipes at roundabouts, people coming in or out a lane right into a car already in the space they wish to occupy or via a shunt from behind when the car in front has braked - DRLs on the front of the car aren't going to prevent someone driving in the back of you or make it any less likely, changing lanes straight into the side of someone isn't going to be prevented by DRLs either. If the person colliding can't see the other car's DRLs prior to collision then they aren't going to help.
I'm sure the fuel companies would encourage it to reap the rewards of increased fuel usage (albeit 0.9% according to the report you linked to, not huge on an idividual basis, but noticeable collectively), one of the bones of contention for some of the EU member states that opposed the introduction of DRLs.