I couldn't say whether it's the most important skill, but I believe that it's definitely a required skill set for anyone that carries a gun for defense.
To wit:
Having assisted a friend of mine while he teaches a fundamentals course (mostly I help set up target frames and clean up, but I also handle some of the camera duties and I'm available to the students to answer questions they might have, and of course helping to ensure safe practices during drills), I've seen students that obviously have never spent any time drawing their weapon and executing a drill.
I've seen students fumble the draw so badly they sometimes dropped their weapon.
Others that took more than 5 seconds to draw and begin firing, students that muzzled themselves and others while trying to re-holster their weapon.
And so on.
Many of these students could shoot acceptably accurately once they got on target, and if the BG wasn't looking at them while they tried to get their weapon pointed downrange, they'd be okay with their 5+ second basically incompetent draw stroke, probably.
However, how about his scenario (this actually happened, except no good guy with a gun was present): you and your family/friends/date are having a bite at a local diner/restaurant. You excuse yourself to use the restrooms, and a few minutes later you return to the table.
As you are walking back, you hear gunshots inside the diner.
You come into view of your table to see the shooter standing just a few feet from your table. He's just finished shooting a couple at the table next to your party. With your family/friends/date cowering underneath/behind their table, People are screaming, crawling, running. The shooter then turns towards your family/friends/date as they cower underneath/behind their table. But you are slightly behind the shooter, and this angle gives you a safe shooting angle. He's likely going to fire his weapon at your people in the next few seconds.
Now how important is it to get your gun out of its holster and firing in the least amount of time? I guess it depends on how much you like your family/friends/date.
I've also watched my share of YT gunfight videos. I've been paying particular attention to the failures. I've noticed that a number of these "good guys with guns" fail because they have issues with getting their gun into the fight. Not most of them, but a fair percentage.
Of course, I'm not saying that they would've won their engagements had they drawn their gun "lightning quick," but at least they might've had a chance.
Now I don't have a "lightning-quick" draw stroke, but I practice my draw stroke 25 times every morning, 5 days a week. I average about 1.2 to 1.3 seconds from buzzer to first shot, from polo shirt concealment. That's not going to be fast enough to win any tournaments, but it's quick, and more importantly, it's consistent; it's basically become instinctual at this point.
Now, as others have said, the majority of the time, a super-fast draw isn't probably going to come into play in a "typical" gunfight, but history shows us that sometimes it is a useful skill to have.