A good grip can't hurt...
... and I won't argue that it will.
I also have not yet had to defend my weapon against an actual BG, intent on taking it from me.
But I do practice retention and takeaways with training shapes (wooden or rubber pistols, knives, clubs; PVC pipe, etc) on average probably at least once a week. (Schedule and opportunities allow for about three mat sessions per week, the last few years)
Grip falls way behind several other factors:
1) Awareness
2) Ma'ai (relative position and distance between self and attacker)
3) Kizushi (own body position and balance)
Given enough awareness to respond to what's happening, then the things that are really going to affect ma'ai and kizushi are your feet and your hips. Trying to do too much with hands or upper body strength when somebody rushes you is a pretty good way to ensure the faster and/or stronger guy wins the confrontation.
OTOH, moving off the line, and knowing how to pivot while maintaining your own balance, will enable you to negate an awful lot of attacks.
Let's say you have a good, strong grip, but you're off balance. The gun may not leave your hand, but is your wrist strong enough to resist both the other guy's hands? Odds are, no.
But if you pivot in a way that moves your body partially between the attacker and your pistol, you make him sacrifice his own balance to make the grab.
Without meaning to introduce too much veer, here, if you look at melee weapons as opposed to firearms, some takeaways/takedowns actually work better if the armed person keeps a grip - because the weapon then gets rotated to lock him up.
With training pistol shapes that have trigger guards, one of the things we have to be very careful of is not breaking the trigger finger of the guy holding the shape.
If you are really concerned that you might find yourself in a weapon retention scenario, I very strongly suggest you find a place that offers hand-to-hand training with an emphasis on weapon retention, and find out for yourself what does and doesn't work.
Talking about it, or reading about it, isn't the same as running hundreds or thousands of repetitions. We're all shooters, and should all understand the importance of muscle memory (and balance, and movement, and awareness).
Regards,
M