If you look very closely at Steve Rajeff's casting grip during his cast, I think he is using the key grip in which you hold the rod in your hand like you would a key. The key grip is recommended by Mel Krieger, who taught Steve at the Golden Gate Casting Club. His thumb is not on top but on the inside of the grip with the index finger wrapping around the outside. Now if you hold a rod like that and instead of wrapping the index finger around to the side, you extend it to the side, you are very close to the 3 point grip. The point being that the best casting instructor and the best fly caster do not use the thumb on top grip, but a grip that aligns the rod more closely with the forearm
The key grip is also called the palm out grip.
The Casting Grip- Fly Casting School
As Len Zickler says, an accomplished caster can use any grip because he is in tune with the rod position during the cast. However, a common error for beginners is to break the wrist on the backcast. Hold your hand in the palm forward position and try to flex your wrist backward. It is not easy or natural. Now position your wrist in a karate chop position which is the thumb on top position. It is quite easy to karate chop forward or backward. That leads to wrist breaks in casting.
The advantage of the thumb on top position is power but are long casts the object when teaching beginners or is a proper stop location more important than distance? If the proper stop position is the more important, should not one consider a grip which makes that a lot easier, when a caster has problems with wrist breaks?
My point is not to change anyone's grip who is already satisfied with the one they are using; but to have you consider that when a caster has a problem with wrist breaks, a grip change rather than putting the rod butt in a sleeve might be a more elegant solution.
The thumb on top grip is not the favored grip of distance casters who want the most powerful grip. It is the palm out or key grip.