liquidgrey:
I didn't want to get into the "why", but knight65 is correct, IMHO. I ended up with an M&P90C with a safety, but that largely was a "because we have one", and being a 1911 guy, it's presence/need doesn't bother me. My other two M&P's don't have the thumb safety. (The M&P9C is a "house gun" right now - if my daughter ever gets off the dime, it's hers, for her apartment.)
You DO need a 1/8" roll pin punch, and the correct sear block assembly, too, but unless you try to remove the sear itself (NOT recommended if you don't need to), it just seems to be a bit magical to get the sear block back into the frame without several tries. IMHO, a half-hour bench charge if you get a local smith to do it for you.
My gunsmithing skills are just above getting the grips back on the correct sides, but with a 1911, you do need to have some skills or a friend who does

.... The M&P series guns are a bit of a jigsaw puzzle, but not at all difficult. (DON'T try to play with the sear assembly on a KelTec P3AT.... It's almost trivial, but seriously counter-intuitive. As in "what were they thinking!".)
IMHO, if you're going to take the thing apart anyway, grab an Apex DCAEK kit while you're at it. Plan on going a little nuts with the trigger block assembly, but it's still DIY, or your local smith likely would install it during that same half-hour....
Regards,