Why would a cop need it??? Simple...survival.
I carry two handguns and two knives, one S&W SWAT folder on my pants pocket (weak side) and a S&W Boot knife on my weak side boot. Also carry a .38 on strong side boot.
In a ground fighting situation where you are "holding on" to your suspect, you never know what boot or pocket you will physically be able to get to. I figure if it becomes a deadly force issue involving weapon retention, my strong hand will secure my primary duty weapon while I attempt to gain access to one of the three "secondary" weapons on my person with my weak hand.
If all I can get to is a knife, I'll slice the SOB's throat, sink it into his thigh, ass, neck, anywhere I can...simplt to bring an end to aggression on the suspects part. And you are right...you won't see a cop whip out a knife to fight with, only a fool brings a knife to a gun fight.