Motorcyclists have a perspex screen about 100mmx200mm to see out of. It gets dirty incredibly easy on the outside, and mists ups incredibly easy on the inside. A car using sidelights only in situations where all other car are on dipped beams makes the car look further away if your just doing a quick glance at a junction. Yes your rear lights are on to help people coming up behind you, but people waiting to pull out at junctions need to see YOU and accurately judge how far away you are.
If everyone drove sidelights only then it would be bearable, but for 1 in 20 to be doing it is stupid IME.
Oh, and the whole point of motorcyclists having their headlights on during the day is that it helps them stand out. If all vehicles have their lights on all the time as per the proposed legistation then they will no longer stand out!