Back in the 1970s, the only vehicles that had lights on during daylight hours were Volvos - with sidelights. Then came legislation that meant you should use dipped headlights in poor weather conditions. In fact, under the law, outside 30mph zones, if you feel lights are necessary then you MUST use dipped headlights. Using sidelights alone would be an offence - apart from the presumption that 'daytime running lights' are somehow different :-)
Knowing the quirkiness of English law, definitions only started to appear as a result of case law. So, for example, 'fog' and 'heavy rain' became included as examples of 'poor weather'. However, these two conditions are well described by the Met Office - but this is not known by the general public who assume that anything in excess of drizzle is 'heavy rain' and 'fog' includes mist on distant hills. 'Heavy Rain' is only experienced in this country probably a couple of times a year (remember that the Met Office is essentially international and so its definitions have to span up to monsoon levels!). 'Fog' is a visibilty of less than 1000m (yes a Kilometre!) - although for traffic purposes it can be considered as low as 400m or even 200m.
The result of all this is that more and more people having been putting on more and more lights as the years have gone.
Yes, it is imporant to be seen, but this is now reaching the point where it is counterproductive. So many cars with daylight headlights - desperate to be seen - are creating a real danger for cyclists and pedestrians who are easily lost amongst the rest of the traffic. The situation is being exacerbated by halogen, and now LED, lighting which - being more intense - creates a higher dazzle factor. This doesn't stop when the lights are passed.
The human eye responds in seconds to accomodate increased light levels. This is probably an 'eye-protection' neccessity. However, the reverse is NOT true. After experiencing a bright flash of light, it can take approaching 20 MINUTES for the eye to fully recover. This means that one is effectively temporarily, but partially, blinded for a period. This is not good. Even high up in a lorry cab it is not possible to keep out of the line of sight of too many lights.
So, lets save headlights for when its dark (or as other legislation demands).
The only exception should be for motorcyclists