I have four reasons:

1. It makes the gun generally safer to carry. (Imagine a person with modest experience carrying a M&P pistol with a four lbs trigger in an IWB).*

2. On S&W 3d Gen pistols that have safeties, the safety is mounted on the slide. In that location, it makes it easier to grip and operate the slide.

3. On S&W 3d Gen pistols that have safeties, the safety is also a decocker, which makes the gun safer during reinsertion into the holster. See "Glock Foot."

4. Having to operate the safety takes a very short period of time, but that brief additional time supports the task of gathering data on the threat, the situation, the backstop, etc.; and deciding whether to fire and what to shoot at -- the most under-rated but most important part of this concealed-carry self-defense thing.

*There are some specific scenarios where knowing how to operate a specific safety makes it hard for someone else to operate the gun. There are reported cases of police officers have benefited from bad guys not knowing how to unsafe the cop's pistol (the interweb indicates 10-20% of officers who get shot are shot with their own guns). There is also the recent case of the Mom who was shot in her car by her Shield, wielded by her two year old in the back seat -- a safety might have prevented that.

__________________
S&W Rover