Never....leave this to the gangbangers.
And, too much potential to shoot Mr.Happy off...and not in the good sense of that phrase!
Well, even Massad Ayoob has admitted to carrying thus on occasion. In fact, I had a thread a few months back when I specifically asked Massad about this, and his response was that he has but that he doesn't necessarily advocate it. In any event, he is certainly not a gangbanger.
For me, I Mexican carried for a long time simply because I am a pretty big guy, and it was very hard to find a IWB holster that was comfortable enough for me. By comfortable, by the way, I mean "Not a instrument of torture". The gun itself was comfortable, and it stayed in place just fine. Leather holsters just dug into me so bad as to cause open wounds! Anyway, I finally found an uncle mikes holster that works just fine and now use that most of the time. In the past, I have also made use of the clipdraw, and have been perfectly satisfied.
I don't think that mexican carry is inherently unsafe at all. First, if you carry a revolver with a hammer as I do, you can just put your thumb on the hammer, and there is no way it can fire accidentally from the trigger being pulled somehow. Secondly, a holster is not some sort of "can't shoot myself in the leg" device. It offers a theoretically more secure way to carry a handgun, and in practice there probably won't be anything in it that can trip the hammer, but thats about it. That being said, there is nothing inside my waistband that can trip the hammer either! In either case, care and attention are what keeps you from shooting yourself, not the method of carry.
Finally, I don't know about you guys, but holstered or not my barrel is nowhere near my junk. If it goes off (in either case) I am going to have one heck of a painful leg wound but thats it. If there is one simple rule I have in life, it's this: Don't point a weapon at your crotch. Ever.