The anatomy of the forearm is such that the three highest digits (pink, ring, middle fingers) fuse to a common tendon connecting to flexor digitorum profundus - the main mucle that flexes the finger. The index finger also attaches to this muscle, but via a separate tendon.
These tendon run straighter, through the carpal tunnel, if only three fingers are engaged; the index finger causes the tendon group to deviate. This may weaken the grip a bit.
The tendon to the thumb is also somewhat bent as it goes through the wrist.
Try this - make a fist with just the 3-5 fingers. Feel your tendons in the wrist with your other hand. Add your index and thumb into the grip, and feel how the tendons shift.
See http://www.bartleby.com/107/illus415.html or http://www.emedicine.com/plastic/topic296.htm
With a gi, I prefer the three finger grip; but for no-gi, I like to grab the arm by pinching with thumb and forefinger, just above the epicondyles of the humerus (bony projections just above the elbow).
PS. I seem to remember that some wrestlers prefer the four finger (no thumb) grip when working an arm. Not so sure why.