Mas Ayoob wisely teaches that we should use what works!
When a threat comes from the side, you turn to the threat . . . using the Weaver.
If the threat is in front, you might transition to Isosceles . . . and back again.
Up close and personal, you'll naturally go into the fighting stance of the Weaver again.
If you are wounded, you must transition to a single handed grip . . . either strong or weak hand.
If something gets in your eye you might have to smoothly shift to your non-dominant eye (try that sometime and you'll see why you need to practice hard before this might happen.
Folks, train hard, and train variably! Master it all. It is foolish to remain a one-trick pony.
T.
PS: My favorite grip in most matches is Isosceles. Then again . . . ain't nobody shooting back!!! I'm a big enough target as it is, not to have to turn face-on!