The main thing about flechette ammunition is that it's commonly used for home defense. Frangible gets used a lot too. So even if a group of shadowrunners is unlikely to use flechette or frangible ammunition, their opposition might.
It's also for those niche times where you don't want your bullets ever accidentally going through walls. In fact, they even recommend Frangible rounds in
The Assassin's Primer, as you don't want to kill innocents if you don't have to. So, basically, most of the time PCs are unlikely to use them. However, NPCs, law enforcement (their opposition, as you have pointed out) and normal people might use those types of rounds. You need to have stats even if PCs don't use them, and in the off chance they do, then all good.
I'm still annoyed that Hollow Point rounds are Forbidden though. While it's true that Flechette has probably replaced them, I know that in
my home defense, you use hollow points because you want to put people down if you have to use a gun. You don't want to shoot a shot at someone and have them keep coming at you with an axe, or still standing to shoot you back.
Just so you know, I'm agreeing with you; just wanted to elaborate my thoughts since they were similar to yours.