I have a S&W 642 hammerles because I don't want it to snag on clothing. Also, in a self defense situation my construct would be that the attacker/s are going to be so close that I can smell what they had for lunch. No time for fine shooting and aimed shots.
I'll toss in a vote for models with hammers.
I have a little Taurus 85 with a hammer but have been shopping for a semi. I like a hammer there too, so I have settled on a Sig Pro in 40 caliber (Single/Double action).
The why falls into the same reasons as lazer sights or racking a shotgun slide. It give whatever it is threatening you one last chance to begin think rationally.