The M&P's thumb safety is a trigger block. It won't prevent any sort of true AD (as in "dropping the gun and disengaging the sear"), although the drop safety will do that.

The thumb safety in a 1911 is a hammer block. It won't prevent an AD from dropping the gun, either, but more recent 1911's may have a drop safety (either the Colt design that ended up in the Series 80, the Swartz - pre WWII design that Colt dropped, but Kimber uses, or something else) that covers the problem. The grip safety is a trigger block like that in the M&P.

S&W's "wondernines" and similar designs (my old M39-2) have a thumb safety that functions as a decocker, and locks the firing pin. Pretty much absolutely safe, but the fool thing is backwards compared to the 1911, and some models only use the lever as a decocker. I think they stole this from the Walther people - my PPK/S has the same setup.

We end up with "dropping the gun", which is probably a very unlikely occurrence (when it results in a discharge, that is), foreign materials (shirt tail, for example) in the trigger, and various finger on the trigger issues.

A firing pin (or striker) block (on both the M&P and current 1911's) may be the only way to deal with a "drop" situation, as unlikely as this is. In the 1911 line, a titanium firing pin (extra weight) seems to be the way around this without adding extra parts, if I remember right.

The various magic triggers in the M&P or Glock really don't help much if you get something in the trigger, but a trigger block type thumb or grip safety can help.

The hammer block safety (1911's and the PPK-style guns) may or may not (the 1911's won't) keep you from a "drop" discharge, but we've pretty well counted that out of the problem area anyway.

Overall, IMHO, it's what feels good for you. The thumb safety in the M&P can be "tuned" a little to make it a bit stiffer, or a bit looser. I'd like mine to be stiffer. But I've got one on my usual carry M&P40C because I'm a 1911 guy and just used (conditioned!) to flip off the safety. IMHO, you don't otherwise need it.

"The other guy"? I'm not sure any of us would manage to flip ON a safety, or drop a magazine in a "gun grab" situation, however if somebody's taking the gun from your holster, that's a different animal. BUT, IMHO, that's most likely in a "uniformed officer" situation, which isn't really applicable to the non-LEO. I don't think the average non-LEO need to think about that.

Regards,

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Stu.