As John3200 said, striker fired pistols like the M&P function very much like revolvers. That was one of the selling points when Glock started marketing their firearms to LE agencies. The transition from revolvers to Glocks would be much easier because Glocks were like revolvers in that there is no active safety device in either.
What prevents accidental firing of an M&P is what prevents accidental firing of any firearm. Know the four rules and follow them. Plus, don't leave firearms where kids or other untrained people can get them.
Get what you are comfortable with, but any safety device can be defeated, even by a 5 year old. Safe handling and storage are the real safety devices.
Actually, the M&P is closser in function to a 1911 than a revolver. Now the Glock has a striker that is at the forward "rest" position and as trigger pressure is applied, the striker is moved rearward until it reaches the point where the sear no longer engages the striker, flies forward and fired the cartridge. This is closer to a double action revolver in function but one with a shorter and lighter trigger pull.
Now the M&P (as well as the XD) do not have this double action feature where the striker is in the forward position. In both of these weapons, the striker is at full cock against the sear surface in much the same way the sear holds back the hammer on a 1911. In both the M&P and XD, the trigger be it with a tab like the XD or a hinged trigger in the case of the M&P does not allow the trigger to be pulled to the rear unless the shooters finger (or other object) depressed the tab (XD) or lower portion of the trigger (M&P) that allows the actual trigger to move to the rear, depress the sear, allows the firing pin to move forward and fire the cartridge. The 1911 accomplishes this in much the same way with a grip safety that must be depressed by the shooters hand that will then allow the trigger to move to the rear and fire the round.
What this basically means is that an M&P without a manual safety is more akin to carrying around a 1911 cocked and unlocked than it is to a revolver. Now while we know that shooters "should" keep their finger off the trigger, being a professional instructor, I have plenty of experience with shooters that have been trained to do just that yet somehow still manage to poop the bed and still leave fingers on triggers when they shouldn't be. Just a fact of life that it can and does happen. Even Glocks with an action that is more in line with a double action revolver has scads of stories about unintentional negligent discharges like the guy in the video that shoots himself in the leg, the female cop providing overwatch on the suspect proned out and caps a round off next to his head that was also caught on video to say nothing of the numberous reports (especially early on) about shooters that would shoot themselves in the leg when going to holster because a finger was on the trigger, hit the holster, finger stops moving, gun doesn't, trigger is depressed and a round goes off.
Finally in comparing all of these designs be it a Glock, M&P, XD, etc. to a double action revolver, in most every one of these cases, the trigger on the autos are shorter and lighter than they are on a double action revolver. A DA revolver lacks a manual safety because the longer, heavier trigger pull is less likely to discharge if a finger is left on the trigger. It's pull is heavy enough that you know you are pulling the trigger. Not so with the striker fired autos (short of a NY or NY2 trigger on a Glock) with a shorter and lighter trigger pull. There just isn't that margin for error a DA revolver has.
It's for these reasons that I prefer to have a grip safety on my XD rather than deactivate it, a manual safety on my 1911s and a manual safety on my M&P45 as well.
