I again ask what was so wrong with the "You are surrounded. Come out with your hands up." routine of years past. Instead of anonymous ninjas in paramilitary black raiding a house and shooting anything that moves, use some "due process."
And "due process" is what it's about. Presentation of a warrant, or at least pretty careful announcement of identity. Blasting down doors without presentation of an arrest or search warrant is just not the American way.
Are there circumstances that can ever justify no-warning attacks? I suppose so, such as when clear evidence of, say, a bomb-making or terrorist cell is invovled. Neither condition was met at at either Ruby Ridge or Waco, nor in the vast number of midnight drug raids.
"No Knock" warrants came about in effort to limit the ability of the suspect to dispose of evidence - another result of the "war on drugs." - r.w.