It is 100% legal with an approved BATFE Form 4. Please explain why it's stupid? Because you've watched too many Hollywood movies, and only assassins use them? Since 1934 only one legally registered NFA item has been recorded as being used in a crime. Oh, and my local Sheriff didn't think it was stupid when he signed/approved the forms for all 7 of my silencers, or my SBR.
It allows one to shoot a firearm without hearing protection, reduces recoil, reduces flash, etc. I can introduce new, young, shooters to the sport of shooting without them being jumpy and flinching everytime they squeeze the trigger due to to the typically loud noise emitted from firearms.
By the way, hunting is legal in several states with silencers, as well as machine guns.
About the only gun that is anywhere near "Hollywood" quiet when silenced is a .22lr. Everything else generally is loud. .223/5.56, .308, etc. all still have supersonic crack of the bullet breaking the sound barrier, and sound like an unsuppressed/unsilenced .22lr.