Good points. The three point and forefinger on top are different grips.
The thumb on top I don't like for the backcast from the tracking standpoint that you pointed out, and also because the rod is pointed horizintally on the backcast when the wrist is in a comfortble position. It is too restrictive a grip for the backcast when overhead casting, though not as bad with a sidearm motion since the radius and ulna bones aren't twisted in the awkward position that causes the tracking error you spoke of.
It is a powerful grip for the forward cast though, and that is why so many distance casters change grips after the backcast to the thumb on top for the forward cast.
However, for fishing, it is purely a matter of preference and what works for each situation, same as body position and casting style. For instance, I doubt many people would throw a 13 wt with the forefinger on top for more than one try.
But when teaching beginners, or someone with a backcast problem, a grip change can often make a huge difference.