I'm currently riding a rim brake bike down the same hill I've ridden my mountain bike. This hill has intersections and 4 way stops with traffic entering and leaving. With disc brakes I worried less about stopping and more about how fast i could get thru the intersections regardless of traffic. However on rim brakes Im constantly worried about my speed getting too high and whether or not someone will pull out in front of me before i have a chance to slow down, not stop.
To me its a no brainer and a safety concern because if you don't have disc brakes you're looking for a crash. Disc brakes are a god send.