The big big problem you can run ito is if your used to having your finger on the trigger as part of the draw when you are not on target comes when you reholster. If you like bullets in your boots keep it up but as for me I'll keep my finger off the trigger untill on target.
In a quick draw drill, I happen to agree with you. I personally don't train starting with my finger on the trigger while in the holster. Once I have cleared my holster, and the weapon is facing halfway to horizontal, my finger goes in the trigger guard and in contact with the trigger face, so that by the time I am on target, my finger is already there and beginning to squeeze while I put the front sight on COM. Others follow the Jordan method, and if that's what they train for, and they put their thumb on the hammer, I have no criticism of it. I certainly am in no position to argue with the likes of Bill Jordan.
Here's an example of what I am MOSTLY referring to: You are in an office building. You hear gun shots and people screaming and running in your direction. Your office has no back door, and in comes a man in fatigues holding a pump shotgun, pistol strapped on his belt with five mags in pouches and a bandolier of shotgun shells over his chest, blasting secretaries as they flee, but he doesn't see you. Your double action revolver is ALREADY DRAWN, as it came out when the shooting and screaming started. You are behind cover, but peering at the lunatic around the edge of the doorway, your office lights are off, as you flipped them when you heard the shooting. Soon as he gets within 20 feet or so, you plan to pop him. No cocking, as that would give your position away. Is your finger out of the trigger guard RIGHT NOW, or is it lightly touching the trigger face? Mine will be lightly touching the trigger face, in preparation to best use my weapon to save my life, thank you very much. I will not wait till the last second to haphazardly thrust my trigger finger into the guard as I turn to take aim and fire. Rule number three be damned.
Now, if my weapon were a Browning High Power or a Government Model, manual safety would be engaged, trigger finger would be out of the trigger guard parallel to the barrel, and thumb would be resting on the top of the manual safety, even at this point, and this would not change until I brought it up to actually take aim and shoot. Different story because different weapon. You have to know your weapon, and which rules apply at which times and which do not. It would be totally unsafe to take the safety off and place my trigger finger on the trigger face of my High Power or Government model until I am actually going for the shot. This is not the case with the double action revolver.