The main advantage to them is their adjustability. You can narrow the advance curve easily. R/E is right about the spark scatter though. Look at a timing light at approx 3000 rpms and watch the timing jump around. You have to look at the range that it operates in and assume that the center of that is the average when setting total timing.
I bought the Mallory because I was having a severe spark scatter problem with the distributor that was in the car at the time. The Mallory solved that. And the locking collars are a good idea on any distributor of that shaft design.
