Well, I guess experiences differ.
Rich Ryan (who once ran the largest WC school in the states) taught us a technique called an "impact push" during one of the iCAT camps. Basically, if you get your structure down you can deliver a lot of force into a target with movement from only your arms. Yet, the force isn't actually coming from your arms, your arms just transfer it into the target.
Several of the other guys who I mentioned, who are master level WC/JKD/martial arts guys, also adamantly insist that it is possible to learn how to maintain this structure, even during movement/combat. By learning how to do this, you can learn to develop a lot more power in rapid fire chain punches than most people would think.
Well, yeah, if you are lying on your back then only your arms are supplying the force, but even then it still comes from the ground up.
From a standing position, even if you purposely try to punch powerfully with only your arm power, you cannot (or at least you can't generate any force in this manner, and you will literally be pushed backwards). In order to generate force, you must have something to push off of (for every action there is an equal and opposite reaction), unless you back is against an immoveable object (wall/floor/etc...) then the only thing you have to push off of is the ground.
I agree that you can do this to a greater or less degree though. The lesser the degree, the more you will be pushed back, as you have no support. The greater the degree the more force will go into the target and the more it'll be displaced.
Chain punches are not the most powerful punches, not even close. But if you understand correct structure, they can still deliver decent amounts of force into the target. No, you can't rotate at the hips/knees/ankles/spine like a lot of punching styles teach. But as long as your structure is solid, you can deliver quite a bit of force.
Really all the rotation does is to help transmit maximal force from the ground into the target, it in and of itself doesn't produce force.
The first video that you posted was very low level WC. The guy throwing those chain punches was off balance most of the time and yeah, he has no idea how to generate force during chain punching. I do like the fact that he was actually practicing against a resisting opponent though, props to him for that.
The second one was fast, but it's pretty tough to tell whether or not he understands correct structure. It didn't look like he did to me, but since he's hitting an immoveable target, it's impossible to know for certain.
The third one would not be "chain punching" from my experience. Yeah, you are "chaining" punches together, but those are just combinations. In WC/JKD "chain punching" refers to a "straight blast" or rapid fire straight line punching.
I understand what you are saying, and no chain punches don't necessarily involve the same body mechanics as a one inch punch (though you could potentially start chain punching and then throw a full power punch using those mechanics into the chain).
My damn computer has been down recently and I'm still running it off of a live cd, so I can't view anything but flash videos (which luckily youtube is). As a result I am limited to what videos I can find on the web that would demonstrate how to properly deliver chain punches with power (not to mention that it's not something you see all that often, so even then it might be hard to find).
But here is a video demonstrating just how powerful WC structure can be if mastered. Check out the part towards the end with Sigung Tsui Seung Tin. What he placed on the scale was a mahjongg piece (to really minimize his balance) and the weight on the scale went up dramatically the more the other guy tried to push him backwards.
The second page is from Joel Weinberg's site. Like I said, I can't view the videos atm, but I've watched them before and Joel is incredible with his structure abilities. You can also check out the picture gallery to see some of his structure demonstrations.
Now, like I said before, I'm not claiming that chain punching is the most powerful type of punching, far from it. All I'm saying is that if you really know what you're doing, it's not just the "patty cake" style punching that most people think either.