I see them pretty common. My markdwarves usually take them out before they reach the cage traps. Hell the markdwarfs will take out entire sieges before they reach the cage traps. I had deactivate the markdwarf squad to catch everything I have. I think thats when that minotaur slipped through. I normally use that arena cage to train markdwarfs. I have them do a patrol route around the cage, then loop back to the ammo pile for one of the waypoints. I added the outer wall so fighting stops scaring the haulers. I do that trick where you can disarm weapons but not armor. Keeps the rookie markdwarfs safe from goblin archers. They get more target practice when they still have armor on.
designate mass forbid and mass dump on the animal cage stockpile. Then go to the stocks screen and unforbid everything with a D next to it. That will force them to grab all of the weapons and leave the armor. Once thats done, do a designate mass Undump and reclaim on the cage stockpile again.
I think on my next fort. I am going to make the same thing, but really deep underground, on top of a large 10x10 retracting bridge over the magma sea to dispose of everything super easy without requiring so many hauling jobs. I can just unforbid iron only weapons to claim and flush everything else down with a toilet lever.