Opinions will differ, so I'll toss my 2c into the ring. Okay maybe 3c, this is a long post...
1. Does a choke improve a functional cable
2. make an otherwise non-functional cable functional
It can. The purpose of a choke is twofold. It can (A) prevent interference to the accessory it's attached to, and/or (B) prevent the accessory from interfering with other
equipment. It works a bit like a surge-suppressor, but is two-way
. The accessory may use a "noisy" power supply in its enclosure, and that may cause hashing lines to appear on your monitor (or nearby television) as an example. Or when your laser printer kicks on (and makes the light in your room dim VERY briefly) it may cause your webcam's audio to POP loudly. Adding a choke to one of the cables in the mix can block this interference, or at least limit it. I've seen surges cause gear to reboot or freeze, sometimes consistently, sometimes intermittently, as a result of interference from other accessories, because CPUs that aren't properly protected can do really weird things when they get "blipped". Chokes are very good at stopping weird digital noise from coming out of cheap external speakers when you set your cell phone down on the table near their power or audio cables.
3. decrease or increase manufacturing costs
everything adds cost, but it may be the cheapest short-term solution. There may be arguments that hardening the accessory against interference, or adding internal filtering on it to prevent it from CAUSING interference may be cheaper than the cost of the choke. It's just a matter of counting beans. If you've already got your design sent and 100,000 of them manufactured, and they're being sent to another facility to assemble and package, and the UL/CE cert people tell you that you can't sell it due to too high of emissions, and the cable is coming from another source, it will DEFINITELY be cheaper to tell your cable source to change to a cable that has a choke in it for another 15 cents/unit rather than scrap or fix your existing stock, redesign your accessory, and reconfigure your manufacturing system. (though Revision 2 will likely incorporate changes that make the choke unnecessary to pass certs, and save them 10 of those 15 cents)
Also consider placement. A lot more things have chokes than you are aware of. MOST consumer electronics have them inside their enclosure instead of in the cord. Cheaper gear skips that to save manufacturing costs, and only bundle them with the more expensive cord that has the choke in it when selling in a country that requires lower emissions.
4. do something that's beyond my ability to think of?
I've reused chokes for other things, but most people won't have much luck there. It takes about 9 of them to make a decent balun for ham radio. I've also seen them mitigate the effects of a nearby lightning strike when placed on the AC line cord. (though it appears to be DC, lightning IS a pulse, having many of the qualities of an AC source, and thus can be blocked or limited by a choke)
5. At which end of a cable is a choke [most] desirable, and why?
Interference needs to be stopped as close to the source as possible, or as close to the sensitive equipment as possible. Any unnecessary wire in between the source and the choke will function as an antenna to radiate the signal into the air. Any unnecessary wire between the accessory and the choke will also function as an antenna, to receive and channel the interference into the accessory. Which is worse depends on the design of what's receiving it. Some gear is very sensitive to interference over its power wires and other equipment is more sensitive to radiated RF interference through the air. In general, you want the choke as close to the accessory as possible in either case. That's why you tend to see them right near the connector to the accessory. This unfortunately often creates a stress point in the 1" or so of cable between the choke and the connector. The original black powerbooks had a recall on their adapters for this reason. (http://www.cnn.com/2001/TECH/ptech/07/09/apple.recall.idg/
) I had one of these and it stopped working. I tore it apart and found the shielding wires in the 1" section of cord had been SHREDDED by the concentrated stress and fell onto the table like a pile of steel wool clippings.
6. Is using them overkill in any/some/all instances?
It's about as much overkill as say a dust mask. Is it overkill for me to wear one when I'm outside? Probably. But not if I live in Beijing! So it's only overkill if you don't actually need it, and everyone's need is different. If your accessory is very noisy and you have other sensitive equipment, OR if your accessory is sensitive and you have other noisy equipment, then you need it. Better to have and not need, than to need and not have!
7. Why their particular presence on camera cables?
Cameras are very sensitive pieces of equipment. That CCD contains millions
of sensitive sensors. If even ONE of them gets majorly interfered with, you're likely to see it. And if interference has any kind of a repeating pattern (think "digital noise") then it's pretty much guaranteed to show up on an image as a moire pattern of some sort or other, even if the interference is VERY small.