I have them drilled in all my equipment.
1) Weight is distributed across three fingers rather than two. Simple physics. Results in less finger stress.
2) Axis rotation increases. (And just in case anyone is wondering, yes, I drilled a ball with an index finger test hole and it decreases axis rotation when used.)
3) Ball projects off my hand better. Allows me to have somewhat of a loft game without having to really hit up on the ball. This may just be attributable to the weight distribution portion of the grip.
1) You have one pinky finger position (i.e., in the hole).
2) Extra $5-$10 per ball to have one drilled, depending on your pro shop. I drill my own stuff so I have no cost.
3) Cuts down slightly the number of pin placements you can use unless you're comfortable drilling into the pin, either in whole or in part.
3) Speed. None of the three were really affected.