It depends on where do you grab.
In no Gi environment, the pulling will be a major issue. Without pulling, many throws can not be performed. When your hand control under your opponent's elbow, you will find out that if your "thumb" and "index finger" can "hook" on your opponent's upper arm bone (right at "humerus" that the bone is from thin to fat), you can have much solid grip even your opponent may have sweat on his body. It's one solution for "slippery grip". You will find out that your "thumb" and "index finger" play an important role in "elbow joint" grip.
http://www.google.com/imgres?imgurl=...EwBg&dur=10978
For "wrist joint" grip, the middle finger, ring finger, and pinky may be more functional because your "pinky" can have a tight "hook" on your opponent's wrist.
http://en.wikipedia.org/wiki/File:Hand_parts.jpg