Depends what you mean by useful. This is NYPD we're talking about, these will be super useful for arresting and convicting someone that some cop doesn't like

Tell that to the Grim Sleeper!

An unclean sample won't be useful in court, but it will be useful to the investigation. When that contaminated sample comes back to 4 people who don't match the description, and two people who do, the investigation narrows to those two suspects. When they finally do find the guy, they can go back and re-interpret the rest of their evidence in 20/20 hindsight before taking it to court.

About the only way this guy isn't going to get caught is if investigators are selectively incompetent, and/or the general public realizes it needs to "I'm Spartacus" the investigators.

That seems entirely feasible.

So, many older pistol designs used a barrel in a fixed position in relation to the pistols frame. Due to this configuration, the added weight of a suppressor hanging from the barrel does not impede the mechanical operation of the slide cycling.

Modern pistol designs utilize variations of John Brownings tilting barrel design. Because the barrel tilts during the cycling of the pistol, the weight of the suppressor interrupts the movement of the barrel and slide preventing a full stroke of the slide to eject the spent case and feed another round from the magazine. Not good.

In response, something called a Nielsen device was developed, basically a cylinder with a spring in it that encourages the pistol slide to fully cycle. These are sometimes referred to as a “piston” or “booster”.

Suppressors are heavily regulated by the ATF, and id have to guess that when seeking one on the grey or black market, or manufacturing your own, a Nielsen device is not easy to replicate with crude means, nor easy to find on the street.

TL:DR dude was missing a part that makes a pistol work more gooder with a suppressor.

Source: I gun.

Edit: most of the following comment regarding suppressors was apparently super wrong. Leaving my ignorance up so the resulting corrections in the reply make sense. Just don't stop reading after my bullshit comment lol.

In a normal, unsuppressed semiautomatic pistol gunshot, you pull the trigger, a precise little pin strikes the back-center of a bullet, this causes the gunpowder in the back of the bullet to spark and ignite and explode. The projectile portion of the bullet rides the wave of the explosion at supersonic speed down the barrel of the gun, which determines the direction of the path. There is an initial increase of backpressure of gas between the projectile and the back of the barrel, but semiautomatic weapons make use of this to push back the slide, expelling the spent casing and that gas and allowing the spring in the magazine to push the next round into place for the next shot, also significantly reducing recoil in the process.

Suppressors (or "silencers") work by slowing the bullet down and altering the propulsion gas path. Subsonic speeds means no sonic boom. The downside is that you must manually cycle each round yourself, and you will likely experience more recoil per shot.

Afaik, suppressors are pretty damn hard to legally obtain, so my knowledge of them is a combination of my firsthand accounts with my unsuppressed guns, secondhand accounts of suppressors, a moderate understanding of physics, and some guesswork. I could be wrong about some of this, but this is my general understanding that I carry around with me.

See my comments below. I’m pretty convinced he was manually cycling his slide because his suppressor, whether homemade or street sourced, lacked a Nielsen device or “piston” or “booster”