The trouble with DRLs is that it creates a bit of an "arms race" much like bigger and bigger cars.
If every car has bright lights on, then more vulnerable road users like motorbikes lose the additional benefit of having a headlight on...
In what way? The bike is no less visible as it still has a light on. Some may argue that the bike will not stand out from the cars (don't buy this myself as bike lights are almost invariably higher and appear to flicker because of the additional movement) but it will still be evident that there's a vehicle of some type there.
"highlight everything, highlight nothing"
Thats the term that relates to this. If everything has lights on. you can't differentiate between them.
imagine a bike riding in front of a car at night. You are turning right.
You see 2ish - maybe three headlights. You assume they are all the same distance away. a trick of the light maybe, but its just the one car.
When in fact the bike is closer and closing faster.
You make the turn - and chop the biker up.
Biking at night is tough and scary because of this.
So during the day its much nicer to be the one with a light one, while no one else has. You stand out.
If everyone has lights on, you're back to riding invisible.
Lets take this another step. Volvo introduced the mid window brake light. It reduced shunts by 40%. because it was out of the ordinary. Now everyone has them. I doubt they are effective anymore.
A bike accident results in serious injury - bikers (cycles, pedestrians whatever) need to be as distinct from the norm as possible.