As with most things, take this with a pinch of salt, but the truth is that Ecologistas en Acción, a Spanish environmental group, has published a thorough article claiming that daytime lights on cars are not safe, and that the extra burden on the alternator to keep them on only adds fuel consumption.
The group affirms that while countries such as Italy or Spain (with more light hours than their Northern counterparts) are thinking about making daytime lights mandatory, Austria has decided to abolish the measure because it's unsafe.
What are the reasons? First, a psychological mechanism: the lights make motorists more secure about their driving, increasing confidence, more or less thinking "I can be more careless since others can see me better". Second, weaker elements on the road, such as bycicles and people on foot, won't have such lights and so they might become more prone to being hit by vehicles. Austria's Ministry of Interior confirmed that they will stop mandating daytime lights (during good weather conditions) because drivers are more easily distracted.
Finally, the environmental group says that since electricity on cars is not free and must be generated via the alternator that takes power from the engine, the lights thus increase consumption. Nevertheless, the group states alternatives that should be applied to reduce traffic accidents: electronically limited speed on cars, stricter speed limits, criminalisation of radar detectors (which are not legal in the EU) but, above all, an increase mass transit which they consider always safer than cars.
[Source: Ecologistas en Acción (link is in Spanish)]