As above, crossbows fit into weapon traps!
And actually ranged accuracy is helped by the weapon quality, so that's a legit sniper weapon.
The problem isn't it being useless. The problem is that damn weaponsmith using precious adamantine for something that can be made out of copper and beat an adamantine crossbow any time.
Unless your marksdwarves are actually hammerdwarves, that's far, far far from the case.
That crossbow will snipe things way better than any copper crossbow. Won't bash as well, but whupidedoo; that's what your melee dwarves are for.
My dwarves have a bad habit of making raw adamantine crafts. That ring is so pretty.... Riiiight...
An artifact copper crossbow is better than an artifact adamantine crossbow by virtue of bashing attacks, yet chances are if your ranged dwarves are bashing anything of significant danger, then they're already dead anyways.
I've had marksdwarves be hell bent on being suicidal before. Give them a nice, sturdy crossbow, a mountain of bolts, and a safe 1z up platform with fortifications to shoot through..... but do they shoot?
No. They grab their crossbow, rush down the stairs, out the door, right between the horns of the goblin siegers, and attempt to kill the troll door bashers with the butt of the crossbow. The trolls respond with "bitch, please! My 2 year old daughter hits harder than you!", hits the deathwish marksdwarf in his beard with his billy club, knocking the skull thought the brain, tearing the brain, and sending the deathwish marksdwarf sailing off in an arc, where he then collides with an obstacle and turns into an instagib. Then all the burrowed haulers psionically detect the presense of socks.
This has happened enough times that I have taken to locking the marksdwarves in the guard towers to keep them from playing "we would be heros!" With creatures more than 2x their size, with a flippin crossbow to the face.