When It Would Work:
Never. A big part of successfully strangling someone is not advertising that you're going to do that, while getting a few inches of wristbelt around your enemy's throat requires more co-operation from your enemy than WWE. You'd have a better chance of landing a spinning piledriver. Despite this the patent insanely describes this as an ideal weapon for anyone regardless of size or strength, and particularly points out that it's better than mace or pepper spray for someone smaller or weaker than their assailant. But then, this is a man whose response to attack is to tying his own hands together and LUNGE! He probably considers leaving your legs free as easy mode.
Bat'leth Shopping Bag Handle
Patent Number: 5,509,708
In many states it's legal to carry a concealed gun, but not brass knuckles, and this invention is even stupider and more hilarious. The "Handle for carrying objects and self defense" is a set of brass knucks adapted to carry shopping. Those narrow grooves on the sides aren't nostril-eviscerating snags, they're designed to carry plastic bags. Meaning you'll have to stop and unhook your shopping before striking an assailant, because trying to punch fight with groceries dangling off your fist would be like punching through treacle. Any man will tell you that anything "dangling" is a serious tactical weakness in a streetfight.
The suggestion that the handle could be made of plastic doesn't help: brass knuckles can floor someone, but a fistful of plastic knuckling sounds like a sex toy. Though, to be fair, if you could do something like that to a mugger it would probably disable them long enough for you to escape.
This invention reveals a terrifying set of priorities. When preparing to go shopping, this man doesn't bring a bag, or a weapon, but something which can only be used as a weapon when he's carrying bags (and he has to take the bags off first.) That's a ridiculously specific weapon. You couldn't drink tea at this man's his house without injuring yourself with his Combat Spoon, and every night it takes him three minutes to disarm his Bear Trap/Slippers before going to bed. He also wildly misuses apostrophes, but I guarantee I'm not going to be the one to tell him.
When It Would Work:
Unless you run into a Klingon at the K-Mart this is never going to work, and even then it would have to be a baby Klingon who hadn't just bought a better weapon. Like a can of beans. If people can buy superior bludgeons they can later enjoy on toast, the only hand-held item which would be a worse weapon than yours is the scrotum.
Patent Number: 35,600
The Improved Combined Plow and Gun added a giant cannon barrel to a hand-pushed plow, and inverts the word "improved." It was patented during the American Civil War for the farmer who thought his plow wasn't heavy and cumbersome enough. And that his mid-plowed field would be a vital Confederate objective. You'll notice there aren't any links for oxen to pull the plow, and hopefully notice how that's a very good thing (although it would be an extremely fast way to scatter kebabs over a field full of guests.) Messrs French and Fancher were prepared to add kilos of extra cast-iron to the backbreaking labor of a hand-pushed plow just for the chance of one day, maybe, shooting someone.
It's an absolute tragedy of invention. This was in 1862, when many easy and effective upgrades still hadn't been invented, but their best idea for improving the gun was to add a plow to it. This meant that if the enemy wasn't considerate enough to attack from directly in front of the massive cast-iron plowbarrel, you'd be faster converting to Buddhism and coming back for them in your next life with a better gun than aiming it. To show just how behind progress this weapon was, it was patented the same year as Richard J. Gatling's new gun.
Guess who'd win in a fight. Or anything else.
When It Would Work:
This is for someone sowing rows of plants and expecting to be attacked in a straight line from dead ahead, so it's a tragedy it was patented 150 years before Plants vs Zombies.
For more self-defense systems which get their owners the beatings they deserve, check out The 13 Most Irresponsible Self-Defense Gadgets Money Can Buy. If you've given up on gimmicks and want to turn your body into a weapon, read 5 Insane Fighting Manuals (You Probably Shouldn't Listen To).
Luke McKinney's main self-defense strategies are friendliness, buying drinks, and Down-forward-HK into EX Thousand Foot Kick. He also tumbles and has a website.