Killing a main character (or any character) just to kill them, as you seem to be suggesting, doesn't work for me. The death has to have a significant impact on the plot, preferably to make things harder for the survivors. At the same time, you kill off too many, and you run the risk of what Martin's done (at least for me). His first few deaths shocked me, but after a while my reaction was more along the lines of "Okay, I can cross him off the list."
Kill off too many, and you have to get the readers to become attached to completely new characters. The deaths don't mean anything if you don't care about who's dying, and at the same time, if you start killing everyone, why should I care about a character I know is going to die? Near-death and just-escaping-certain-death is far more exciting for me, plus is an excellent opportunity for character development. There's no character development if the character is dead.
And sometimes I like my heroes to live.