I've given this question some real serious thought since my first reply to it. Do we need .40 S&W?? What a really great question!!
Then I thought.....do we need .357 Sig, or .45 GAP?
Gee after looking at the ballistics for the .45 GAP I really wondered why Glock would bother to even develope such a cartidge :

Really would any disasterous police shootout have turned out any better if the LEOs had used .40 S&W????? Maybe so, or maybe not.
The FBI turned to the .40 after turning it's back on that oversized hand canon the 10mm. This was at a time when 9x19mm was all the rage and rave among police departments and civilians using semi-autos. Everyone seemed perfectly fine with the 9mm until a few high profile shootouts appeared in the media. Yeah bad guys with assault rifles shooting it out with FBI agents armed with only .38 special & 9mm pistols, with a 12 ga. shotgun available. It took too long to kill the well armed bad guys so everyone was brainwashed into believing that the 9mm (and .38 Spec) was no longer good anymore. Hmm....pistols against assault rifles....who do you think had superior firepower??? The outcome would have still been the same if the FBI had carried .45 ACP or if the .40 S&W had been available at that time. Then years later in 1997 North Hollywood.....LAPD armed with 9mm Berettas against bad guys with AKs and FALs wearing so much kevlar I'm amazed they could still stand up right. Get real, if 12ga shotguns with 00 buckshot couldn't stop those guys then why do some folks think a .45ACP or .40 S&W handgun would have??? They would NOT have saved the day either.
There is one old and excellent performing cartridge that has been used by both law enforcement and the outlaws of the gangster era. Yes it's the .38 Super. After seeing the ballistic and it's performance I wonder how such a cartidge could be so overlooked and under appreciated as this one is. Now mainly used by competition shooters in matches, rather than used for personal defense. Does it offer any special advantage to an untrained or under skilled shooter?? Probably not. Just amazed how everyone overlooks some good performers in favor of the latest greatest highly publicized new cartridge.
There was a really good post someone wrote on this forum about a retired NYPD cop on the beat during the 1950s who carried a .32 cal revolver and never felt under gunned with it. Going to the range today and seeing young folks with these new hand canons, he just chuckles and wonders if all this new stuff is overkill. It really does make me wonder if all these hot rod new cartridges offer any advantage?? The .38 Spec served as the predominat police cartridge for most of the 20th century (80 years) and is often still carried today.
People say that the criminals back then in the old days weren't like the bad guys today. They are right, in some cases these old bad guys were much much worse and far more dangerous that these punks of todays generation. Gee think of outlaws like Bonnie and Clyde and their Barrow Gang, Machine Gun Kelly, Baby Face Nelson, and Pretty Boy Floyd. These old bad guys really knew their guns and knew how to shoot too.
A bad guys is still human, no matter what era or how doped up he/she may be. A shot to the head or spine is a shot to the head or spine. A shot to a non-vital area still won't do much to stop many criminals. Tactics, strategy, mind set, and shot placement are far more important than caliber.