I agree with you on the dangling stuff, but the rest, who cares?
The placement of your backup is totally personal preference. As long as it isn't bouncing off the reef and is secured to your body in a place where you can easily access it and quickly donate it to another diver, I don't care where you're putting it. Well, I should qualify that. If there's a chance it may go into my mouth, there are a few places I hope you don't put it, but you get my point.
The necklace thing makes some sense, though it isn't as founded in fact as some would lead you to believe. If there was 100%, undesputable evidence that the one and only best place to put a backup is on a necklace, I'm sure it would be a standard for all the agencies. Especially since that's one more piece of gear they could sell you.
Fact is, there isn't proof that's the best place for it. You'll hear talk of studies about how people will rip a reg out of your mouth. You'll also hear of studies that show that people will go for a brightly colored object mounted in the chest area. Then there's the "they'll do what they're trained to do" crowd. The truth is, no one really knows, so the best bet is to do what's comfortable for you and make sure your buddy knows what's going to happen if he needs your help. Of course that also doesn't mean that things aren't going to turn into a cluster (you know what) if someone is out of air, so you deal with that situation as best you can.
Now I'm sure 27 people are going to jump in and say "one time, when I was diving, this guy was out of air, and he grabbed my primary, and I was like, 'whoa, that not cool' but he did it anyway, and I had to use my secondary" but like I said, there's no definitive proof that an out of air diver is going to do one thing or another.
As for the knife, again, who cares? Is it affecting your dive if some other guy has a broadsword strapped to his leg? There is a school of thought that says you can use a big knife like a little knife but you can't use a little knife like a big knife. Now, I've never needed a big knife while diving, but some people may feel as though there is the chance they'll need to knife-fight some creature lurking in the murky abyss. So if having that big knife makes them feel better, they just think it looks cool or if they do have a legitimate reason for needing it, I'm not going to be bothered by it in the least.
