It's a "save your butt" in one situation and a "get you killed" in another, kind of thing.
All depends on how you look at it and which possible scenario(s) you choose to examine. I've often heard the one about how if the bad guy goes for your pistol you can drop the mag and he can't shoot you, if he gets the gun away from you. This is true.
however, I wonder about the wisdom of this. First off, I wonder, why, if the bad guy is grabbing for your gun, don't you just SHOOT HIM???
Next point is, ok, you dropped the mag, now the bad guy can't shoot you with your gun. NOR, can you shoot him! No free lunch there, fellows...
Another point, if you include the rough and tumble, something might hit you mag release button without you realizing it. If this happens in a pistol without the magazine disconnect, you still have one shot. Which might just be the one shot that saves your butt, or someone else's. If you have a magazine disconnect, you have NO shot when you expect one.
I don't see this as a positive feature. Also, worst case the mag fails, gets damaged or lost and you still have ammo (say, loose in your bag or in a box).
If your gun doesn't have a mag disconnect you still have a functional firearm. Slow, single shots but it will still shoot. Lose the mag from a pistol with a disconnect and you do not have a functional firearm. Not even a single shot one.
balance that against the possibility of it preventing an accident. For me, I'd prefer not to have one.
It may have been a feature of some small pocket pistols, I'd have to do some research, but I know the first major service class semi auto that had one was the Browning Hi Power.
According to "legend", the French requested it. FN wanted to sell pistols to the French, they had the unfinished pistol JM Browning had been working on when he died and wanted customers for the gun. The French wanted a magazine disconnect, FN designers finished the Hi Power with that feature (among others Browning didn't create), and then, the French found some reason not to buy the guns. Other people did, and the Hi Power with its disconnect became part of firearms history, serving well in peace and war. (fade out, cue theme music....roll credits...
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Some people think they are a good idea, some don't. I think they're a pain in the butt, but that's just me.
If you think its useful, fine. If you're ever in a situation where it endangers you, you'll probably change your mind. If you're ever in is situation where it saves you, you be a believer for life. All I know is, that if you don't have one, it can neither harm nor help you. and, if you do, it MIGHT.