The intensity of LED DRL’s is fine during daylight hours. However, they’re likely to dazzle oncoming road users after dark if they’re on with the headlights - hence why they turn off when you switch your headlights on.
I may be wrong, but I think that some car’s DRL’s do stay illuminated with the headlamps on - they double up as sidelights. However, they then operate at a lower light intensity (than during daylight hours) so they don’t dazzle.