For me it is the follwing:

1. The dead prop has the least amount of thrust shortening the roll-out.

2. Many field hosting the competitions are very small. I consider it safer when landing in close proximity to myself and the judges to kill the motor.

3. Once the field is made, no problem. Taxiing is nothing but unecessary wear on my Kevlar fuselage and I'd rather have my airplane carried by my caller.

3. An airplane sitting with motor shut down will never nose over or ground loop or have any other, however remotely possible idiotic freak occurence, happen to end the day.