You appear to be the rarity, though. Too many people condone this military-bullshit when it isn't necessary -- and no-knocks are absurd. There are ways to serve warrants that would allow for protection of evidence AND do away with no knocks. For example: Simply turn off the water to a given street or house first. Flushing is impossible.
The water in the toilet tank is used to flush. Shutting off the water supply only prevents the 2nd and 3rd flushes.
But the key question is whether preservation of evidence is worth endangering peoples’ lives. If they are raiding OBL’s compound in Pakistan, sure. If they are raiding US Citizens looking for a little dope, probably not.