i don't know why m&p has 2 piece trigger if it does not have trigger safety like glock/xd?
i think grip safety is fine, remember trigger is 6 pound pull, if your hand is hurt can't hold on to the back of the grip, what do you expect to pull trigger against? same thing with slide, how do you plan to pull slide without solid grip on the gun? it's not like slide is as light as a switch, it's a long hard pull, if your hand is injured such that you can't even hold grip with such small force to depress grip safety, then forget about cycle slide or pull trigger, better option is to run
in fact i would think trigger safety is not as useful as grip since what's the chance of trigger got pulled only by the edge, but spare the center?
imagine you put m&p, glock, xd(m) all inside a tool bag with lots of items and bag get dragged/tossed around, what's the chance of misfire? m&p (without manual safety one) will go off first, anything hold on to trigger will fire, glock 2nd, anything hold on to the center of trigger will fire, xdm the 3rd, anything hold on to the center of trigger AND something pressed on grip safety AT THE SAME TIME will fire, the chance is much smaller than the other 2. conclusion, xd is safest.
and if you ever need to pull trigger, 99.99% you need to pull trigger against back of the grip (6 pound force remember), so grip safety will be naturally depressed, unless in some weird case the gun is glued/fixed solid somehow you can trigger without touch the grip, but in that case how can you aim?