There are a number of cases where law enforcement officers have been saved from death or injury because they intentionally dropped the magazine from their gun before losing control of the gun to an attacker. In at least one case that I know of, the attacker himself dropped the magazine out of a pistol while pushing on buttons, trying to figure out how to off-safe a Smith and Wesson pistol he had taken away from an officer. Indeed, Smith and Wesson has incorporated this feature in all of its semi-automatic pistols, beginning with the Model 39 back in the 1950s, and continuing through all the subsequent model and design changes used in police service since that time. At least, until now.
Their latest (and I think best) semi-auto service pistol, the M&P, makes the magazine disconnect available as an option, rather than a mandatory feature. I can remember many years ago that the Model 39, 59, and later the other variations of their all metal pistols were touted for the fact that a police officer could safely disable and store his pistol when he was off duty by simply removing the magazine. Still, he could immediately put it back in service by inserting the magazine. I wouldn't advocate leaving any gun with a live round in the chamber when it is being stored, regardless of the safety features. In that sense, however, it could be useful in an administrative law enforcement setting, such as checking your gun at a jail or at a courthouse, eliminating the need to remove and re-chamber a round.
Otherwise, there are at least two negatives about the magazine disconnect. The first is that the magazine release can accidentally be pressed while going about your daily routine. If this happens, your gun is disabled and you don't know it until you need it and the magazine isn’t there, or drops out as you draw the gun. The solution to that, of course, is to rearrange your gear so that doesn't happen. Regardless, you only have one round in the pipe if that happens even if the gun does not have a magazine disconnect, so it is important to always have a spare magazine handy to do an instant reload. Speaking of reloading, that is the other negative that is often mentioned in connection with these safeties. What happens if you have a round in the chamber, you’re in mid reload, and you need to fire that round?
Frankly, I think this is a non-issue. The argument started back in the days when advocates of the big bore 1911s were feeling challenged by the high capacity 9mm pistols, mainly the Browning Hi Power and the Smith and Wessons. Both of the nines had magazine disconnects and they were gaining fans. The 1911 crowd was trying to find anything they could to disparage their rivals. The big bore fans advocated shooting until there was one round left in the chamber, then reloading with the gun still hot. This was a faster reload (no slide manipulation) and kept one round ready, just in case. (Yes folks, this was actually taught back in the 1960s and 1970s.) Although this could work on the range, where counting to seven without the stress of someone shooting at you was do-able, it failed miserably under stress. In a real gunfight, the first clue most people have that they need to reload is when they realize that the slide is locked open. Experience has shown that beyond the first 3 or 4 shots, no one can keep accurate track of the number of rounds fired. So if the gun is locked open, it really doesn't matter if there is a magazine disconnect or not. What does matter is your ability to reload quickly and smoothly and get back in the fight.
What about the so-called tactical reload during the lull in the action we often hear about? You would have a live round available in the chamber during that process, just in case someone suddenly jumps out at you. Well, first of all, if you need that round you didn't do a very good job of identifying the lull. And even if you didn't choose wisely, how long does it take you to reload your gun anyway? Two, maybe three seconds? That's a pretty small window of opportunity for your opponent, wouldn't you say? If you are looking for a reason not to have a magazine disconnect, this isn't it. And whether your gun is so equipped or not, you should always have at least one spare magazine.