Well, suppose you were at home watching the tube with your wife/girlfriend when suddenly your door gets kicked in and a flashbang goes off... You're disoriented, your wife doesn't know what's going on, suddenly you have a boot in your neck, an MP5 in your face, your retinas are burning, and all you can think is "WTF is going on?!?! What did I do? Who the hell is in my house?" There was no knock, no forewarning, just all of a sudden you go from making time with the lady to face buried in the carpet with no idea of what's going on. Then they say they "think" you may have done something.
Essentially, my point is if you give them an inch, they'll take a mile. What's to stop them from abusing even EXTREMELY limited circumstances for a no knock warrant? They already abuse enough things, why give them another? (Remember the Magna Carta).
You're so quick to condemn the Hitlers of the world, you're so quick to condemn Nazism (aka Fascism, which, for the record, I despise), but then you advocate it in the form of no-knock warrants!
In closing, we simply can not give agents of the state and/or country that much leeway, we can not give them that much power. After all, all power is supposed to be inherent to the people, and if WE relinquish too much power, we will be left powerless.