I have a '96 850 and the instrument lights don't come on using just the daytime running lights.

However, I'd suggest that you turn your headlights on at night. Although it sounds like you are seeing fine at night using just the DRL's, remember that your taillights also don't illuminate at night when using just the DRL's. Which means that unless braking, you are less visible from behind to other drivers.

Also, police officers are typically trained that a driver who is driving with lights off at night is a sign of a drunken driver, so you could end up getting pulled over needlessly and might be ticketed for driving with your taillights out.

I find the DRL's distracting at night because it's not unusual for me to start to drive at night and then think "why are the instrument lights out?" and then remember that I need to turn on the lights. It's an easy mistake to make, I've done it 500 times if I've done it 50. Cheers.