I'm with the other people who've trained on this:
The reason you don't fight if you've learned a martial art is that you almost never really *need* to fight. Unless someone is threatening your life, someone else's life, or bodily harm, there are always other options. One of the first things drilled into me in my dojo was that fighting isn't good. Yes, it sounds a little cheesy, but it's about being able to defend yourself, as well as the athletic and mental aspects, far more than it is the utility of fighting.
If a guy is mugging you, give him your damned wallet, Ipod, or anything else. Unless he's made it clear that he intends to kill you, money isn't worth the fight. Ever. The people I've seen really get into fights are people with either no or minimal training. The kind of person who'd pick a fight in a bar over a woman is the kind who's never had good training.
Beowulf DW said:
I've never had to use my training in actual combat, but I know people who have. One of the seniors in my old Tang Soo Do class was attacked by some guy on a city street once when he was walking home from his college classes. The guy pulled a knife on him, but before he got to attack, my senior had dropped his books and assumed a front stance. The would-be-mugger got scared and ran off. I suppose the idea of a "victim" who is willing and able to fight is not very appealing to most street thugs.
I wanted to respond to this for a moment. I don't know if Tang Soo Do does things differently from Aikido and Shotokan (the two things I've trained in), but I've been trained first not to fight back during a mugging, and second not to drop into a stance in that situation. If it was just for intimidation, that's fine, but it's bad strategically. If you put your opponent on his guard and he's anything other than an idiot, you've given him an advantage.
On the "grip the slide question":
You're both half right. Gripping the slide before a shot wouldn't help anything, since the slide only moves due to the recoil of the round being fired, so it would still fire. But, you could prevent it from chambering a second round, which might be worth something if you're willing to burn your hand pretty badly. That said, the best method to defuse a gun threat is the same as that with a knife:
Control the gun hand, make sure it's pointing up and away. Breaking the trigger finger helps a lot, because it's downhill from there. Don't try for anything fancy like disarming him, you're likely to either get shot yourself, or get someone else shot. If all else fails, control the barrel of the gun itself (as a very, very, last resort). For all guns, if you can interfere with the hammer falling, you can stop it from firing (every gun uses a hammer), but that takes a lot more chances.
But, I'll reiterate: there's almost never a reason to fight. Be ready to part with everything on your person before you're ready to actually fight. Unless someone just wants to hurt you or someone else, there's no need to fight.