I agree that proper toe-in is something determined by experimentation. This is the case with ALL aspects of proper speaker placement in a room.
As for a reason for extreme toe-in, this can be useful if you are attempting to widen the area that one can sit in and still have some stereo imaging. With extreme toe-in, the listener who is pushed closer to the left speaker will now be on-axis with the right speaker and off-axis with respect to the left speaker. That means that the closer proximity to the left speaker is somewhat compensated for by the more direct sound of the right speaker. The opposite holds true for the listener closer to the right speaker.
As for a reason for extreme toe-in, this can be useful if you are attempting to widen the area that one can sit in and still have some stereo imaging. With extreme toe-in, the listener who is pushed closer to the left speaker will now be on-axis with the right speaker and off-axis with respect to the left speaker. That means that the closer proximity to the left speaker is somewhat compensated for by the more direct sound of the right speaker. The opposite holds true for the listener closer to the right speaker.