There's been some good points raised and some manure thrown. Briefly: there hasn't been a documented case of a magazine disconnector/safety causing a casualty in the US [certainly there have been no lawsuits filed by "victims" or their survivors], there have been quite a few people documented as having been saved by one.

S&W makes the disconnector optional because there are those who refuse to buy a weapon with the feature. Their choice. .

FWIW, I've seen nigh onto 2 million rounds go downrange from disconnector equipped S&W autos without any failures as a result of mechanics. The S&W designs do not require the magazine to be locked in place to fire. Failure to fully seat the magazine generally results in bang, whoops! Corrected by the same immediate action drill used in non disconnector equipped weapons where the magazine wasn't fully seated. Should you fail to seat the magzine sufficiently to activate the trigger, the disconnector probably prevented the magazine from hitting the deck after firing the chambered round. An event much harder to recover from than if the magazine is still in the well. Note: if there's a round in the chamber, you're doing a tactical reload, which is done from behind cover during a lull in the action. Where exactly is the threat?

I started out as an opponent of the device, accepted it as a necessary evil and have become a fan over almost 20 years.