There was a time when most holsters were made with exposed triggers. Then someone got the idea that if they covered the trigger that people wouldn't be putting their finger on the trigger during the draw. It was a 'solution' for lack of proper training.YA side note: I don't quite fully understand these holsters that expose the trigger. Would you mind explaining to me the reason or utility of this? I would think it would be wise to cover the trigger.....
What was induced tho with a covered trigger is more ND when holstering. Those who don't train properly and keep their finger on the trigger when holstering can cause a ND. Additionally if something does get wedged on the trigger while holstering, like the tabs and adjustment gizmos on the bottoms of some jackets, then those can cause a ND. When holstering with a finger on the trigger the finger pushes against the part of the holster and can cause a ND. The tabs and adjustment gizmos getting on the trigger and causing a ND has been well documented. With an open trigger that doesn't happen.
So if you're poorly trained, or like most people have no training, you can take your pick for when the ND can happen. You can have the ND on the draw with an open trigger or a ND on holstering with a covered trigger.