Its NOT just about 2 knuckles…in WC or any effective Martial Art.
We don’t just strike with the 2 bottom knuckles, we drive the entire fist thru. People who focus on which knuckle(s) have to hit first are missing the point, its about proper alignment of the enire body, which is really what we’re striking with, certain strikes transfer this energy and power thru different parts of the hand, foot, body, etc. In a verticle fist, depending on angle, target, etc, the lower 3 knuckles are aligned and the entire fist penetrates/transmits force. If the strike is mostly hitting with the pinky knuckle then its probably just a superficial grazing strike anyway. If you slam that knuckle against a hard object you very well may damage it. If youre just thinking about it static with your fist all tighened up and knocking on a solid surface, then yes you i guess that could hurt your self…but thats not the kind of energy or strike we’re using. With proper conditioning you can transfer lots of power thru that part of your hand without damage. And we strike from many different angles with many different parts of the fingers, wrist, joints, knuckles, hand, elbow, shoulder, knees, shins, feet, etc, etc. Its all the same principle, not being all tight and ‘knocking’ on a person, jong, door, whatever, its relaxed, dynamic, explosive, and penetrating, using proper form, energy, physiology and alignment. More like a hollow point bullet blasting energy through. Its hard to explain these concepts quickly in person let alone in a short paragraph…thats why it takes years to develop this stuff, right?
When you use different strikes such as a horizontal fist (I’m not nessesarily just talking about Wing Chun now) then the physiology and alignment is diffent, youre not going to be aligning your fist the same way and the first part to make contact might be the index-finger-knuckle and middle knuckle, and you can transfer lots of power thru these too, its just a different strike with differnt alignment but the principle is the same.
With boxing gloves on its hard to not get tangled up if you try to stick to just verticle punches (which could be a bad idea anyway), to much big pad on the centerline and youre hands can get trapped easy and trip over eachother and i find it more effective to allow yourself the option to ‘float’ the elbows a bit and use vertical punches, or an angle somewhere in between, to get over the opponents defense, gloves, etc, especially when stiking to the higher targets, head, etc. You can still use vertical fist strikes effectively, especially to the body, but its more difficult to use some of the traditional WC basics that we would use barehanded or on the street. I believe thats at least part of the answer to why boxers started using more horizontal punches when gloves were applied, it evolved more into a sport as the equiptment altered the dynamics and possiblies and there became a more limited spectrum of what can shoot thru effectively.
A verticle fist IS inherently more solid a ‘platform’ in a static situation, just root in a stance, have someone in front of you push youre fist back towards you in the vertical and horizontal positions, and see how its easier to use more of your body and stronger muscle groups with your elbows down. So i think this is a good fundamental punch. Its a very important basic concept and tool for your arsenal. But I wouldnt say to only punch like that or that theres only 1 best way to strike, fighting is dynamic and when you develop youre fa jing and body dynamics, you can transfer good power from vertical fist, horizontal, etc.
Damn, now i’ve typed way more than i intended to starting off, but its a good question and a complex answer, that has more to do with transfering power effectively with your body, then worrying about lining up a certain knuckle, unless of course youre striking pressure point or small target, but then you wouldnt be using a basic straight vertical fist. Hope I put a dent in explaining this rather than just made the issue more confusing. I wish i could better explain this or demonstrate what exactly I mean.

-MutantWarrior
“Far better is it to dare mighty things, to win glorious triumphs, even though checked by failures…than to rank with those poor spirits who neither enjoy nor suffer much, because they live in a gray twilight that knows not victory nor defeat.”
-Theodore Roosevelt