While watching pros play (MLG, GSL,etc) I notice that sometimes a player will move a probe/drone/scv right through the other player's mineral line. I have yet to see a pro try to kill this drone as it passes through.
When someone does this to me, I often have my drones all attack the scout, killing it pretty much instantly. I am only gold, so if I can manage to do this often, I am sure a pro could. Is there a reason they don't? I know I lose mining time, but I feel that I gain by not having to worry about what that scout is doing, esp if it is a probe or scv that could be used to harass my early hatch.