I feed live to some of my snakes, but it has to be done with caution. I feed live because I bought them eating live, and I am trying to convert them to f/t. I think the main concern with live feeding is an injury, which has happened to me on two occasions. Both times the rat had bit the snake after it had coiled. I had to physically remove the rats jaws away from the snake until the rat was dead. It was minor injuries that you have to look for to notice. I put polysporin on the wound and it heals great.
I think the main reason people feed live is convenience (it's just an opinion, it may not be the truth). I reason this because I find most people who feed live breed the feeders themselves, so there is less processing of the rat.
I strongly believe that it is reasonably safe to feed live rodents to snakes as long as you watch the snake until the rodent is dead. Until the prey is dead, it still poses a possible threat. The real problem comes from people who leave the prey unsupervised.