Cheapshooter: "They do If you don't have them!
Why add an additional, small, easy to misplace, and essential part for the small if any advantage of 9MM over 38 Spcl?"
Because they make the loading and unloading process faster and LESS complicated. Moon clips are only easy to lose when they are not full of brass. Are loose rounds easy to lose? Certainly easier to lose than a fully loaded moon clip and harder to load than a fully loaded moon clip. A fully loaded moon clip is no easier to lose or misplace than a loaded magazine for a semi-auto, a speed strip for a revolver, or a speed loader. Do you just carry multiple guns instead of carrying a reload because it's less complicated than carrying a magazine or a speed loader? If your answer is that you carry loose brass, I'd like to suggest this radical new technology that allows your brass to stay together in an easy-to-find bundle that kids these days are calling a moon clip.
If I ever bothered to own a .357 or a .38, I'd definitely get it cut for moon clips, because there is zero disadvantage, because even luddites like yourself can ignore the moon clip feature and insist on loading them individually like a proper gentleman and never be bothered by the myriad of complexities of the unlockable puzzlebox that is the conundrum of the Moon Clip!
And when a fully loaded moon clip is inside your gun, it is pretty durn easy to use, too, no more difficult than a regular revolver, unless pulling a trigger is somehow too complex as well.
Full-price moon clips are $4 from TK Customs; there are cheaper ones on the market that are cheaper still and supposedly (from other TFL users) work just as well. For semi-autos, magazines are typically $20 to even $50 a magazine. Speed loaders are more expensive than moon clips. If you lose a few moon clips, so what? It won't cost you your life, because the only real way a moon clip is easier to lose than a magazine or a speed loader is when it is de-mooned, and you can lose 5 expensive moon clips for the price a cheap auto magazine. I've never lost a magazine OR a moon clip, but however I have certainly misplaced unfired lose brass. To each his own, I suppose.