If you need full beam doing 30mph then you really shouldnt be driving as either your eye sight is not up to scratch or your headlights are defective/dirty.
Yeah, riiiiiiight. Driving anywhere, with lots of dark areas, obstructions and corners to block a dipped beam, putting the mains on will pick up things like pedestrians walking without lights, or reflective clothing, or cyclists doing the same, who would otherwise be concealed by the shadows cast by dipped beams. I do wonder how many people actually drive around any inhabited areas that have no lighting for this not to be obvious. The road I live on in Chippenham has dreadful street lighting, there are cars parked on both sides along almost its entire length, and there’s a bend partway along that only allows one car through because of the parked vehicles on both sides. Late at night, there can be cyclists without lights, peds who will just walk out into the road, etc, and they’re often obscured by the shadows cast by my dipped lights, so I’ll often put my mains on, just to see if there’s anyone around, then I’ll dip again.
On my way out the other evening along an unlit lane I nearly hit a couple in the road who were all in black, no lights, it was only because I flicked the mains on I saw them a few yards in front and could steer away, they were effectively invisible!