Seems like a simple concept to me to keep the finger and foreign objects out of the trigger guard, but hey, what do I know.
So, would you carry a 1911 cocked and unlocked? I mean, just keep your finger and foreign objects out of the triggerguard.
Neither of my M&P's have a manual safety but I really think it is over the line to imply that someone who prefers to have one is per se a sloppy gunhandler. If you train with the gun, it isn't a problem to have a manual safety. I carried a 1911 as a duty gun for a while and was just as fast with it as with the M&P or a glock.
IMO, as long as you follow the rules and train with it regularly, it is purely a matter of preference whether you want a manual safety on a DAO gun like an m&p. That is especially true since glocks/xd's/m&p's are not true DAO guns like centennial j-frame as their action is really set to half-cock as part of it cycling. That why, unlike true DAO's, you don't have second strike capability and your trigger pull is both shorter and lighter than a true DAO.
Best,
Joe