Full-scale races are decided at the finish line. Why should slot car races be different?
Running by laps isn't the answer.
In real racing you do not have track calls. On every track call you would still have the potential for an unfair advantage to the guy that by chance was on a high speed part of the track.
Power off at finish is only one of many stops that we employ at slot-car racing. Even if you eliminated track call's we still race in "stints" on a number of lanes that needs to be added together in one fashion or another.
This also leeds to another can-of-worms. It was briefly touched upon in this thread.
When you change lanes, do you move the car to another track perpendicular to track (Straight over), or do you move to the same numbered segment on the track.
My take is, since we do not see cars spacing in and out when doing close racing, it is fair to say that we can move the cars perpendicular to the track.
It is also the easiest procedure to handle.