Train wrong, and you will do wrong. Period.
In terms of defensive handgun deployment, getting the handgun into the fight - if you are unlucky enough to find yourself in one - is key to your survival. The problem is, most of us (yes, even including me at times in the past) are practicing techniques that might not meet the “stimulus/response” test.
The great benefit of practice is that it makes you better at what you practice. The downside to practice is that it makes you better at what you practice.
Read those last two sentences again.
You will perform like you practice because of a process called “myelination” (Google it). We write programs in our brains by doing something repetitively. The problem is that the program we write might not be the one we want to run in certain circumstances. A skill program is technically not possible to delete once it's been built, even if it was incorrect.
We want to make sure to write the correct program, and stimulus/response is part of this equation.
So how does this relate to defensive handgun deployment?