Back when the Semmerling was marketed things were quite different. If there were other guns of a similar size, you'd be talking about a Derringer or something on that order. And 9mm was typically not anyone's choice in a small defensive gun in the early 80's -- the reputation of 9mm even in full size guns with double stack magazines was not a grand one. This was before the complete waterfall of dinky .380 pistols of the last 5-10 years as well.
The Semmerling was likely to be a hidden gun or a last ditch back-up gun. Compare it to a Derringer or today, maybe an NAA mini-revolver. You don't have a fast second shot (Derringer) and you don't have one-hand operation to your 2nd shot (mini-revolver) but you had extremely small footprint,
.45 caliber and five rounds on board.
To carry one today would be to flip the bird to fiscal responsibility when you consider the (almost) irrational money these pistols bring. But to see it's utility, you have to imagine a totally different landscape of products on the market. For size and operation -- compare the Semmerling to the pistol in my avatar.
All of sudden, it looks like pure genius. (however... at under two bucks - the Liberator is it's own legend)
Honestly, I don't find the idea of a manually operated repeating handgun to be ludicrous. If you only ever cocked a single action hogleg with your second hand, it's a similar rate of fire and a similar payload of ammo -- this one is arguably easier to load and far easier to eject spent brass.
