I'm a Glock guy. I like the fact that I pull the trigger, Glock goes boom. I've become so accustomed to being safe by not putting my finger in the trigger guard that I only engage the safeties on firearms that are not for self defense. In a stressful situation, the last thing I need to do is add yet another step to the equation.
Murdoc has a video that highlights why I'm like this
The gun didn't go off because the safety was on
That's the kind of thing that scares me. When you go into fully-automatic mode, the instant something goes wrong, you have to snap out of it and "think".
The shotgun's safety is the fact I have to rack a shell. The rifle's safety is sending the bolt home. Since those are my two main home defense weapon (screw a handgun!) I leave the safety in the "Fire" position. The last thing I want is some mechanical doodad preventing the hammer from going down when I need it most.
This, of course, is only my opinion. Feel free to chime in with your views.