Ah, so it's just a typical finicky DF function then?
It sounds like being equipped with melee weapons does not preclude them from using their ranged weapons and vice versa, but it can be unreliable.
I wish I had archer towers, but I am still building things up, and it is taking a while to finish projects... particularly when the builders are constantly getting interrupted by the very things I am needing to shoot! Even if I had archer towers, they may not have been in the right place to fix the issue. I had little choice but to use missiles on mobile squads, as my entire above ground operation was being disrupted by giant butterflies who would not come down and fight my melee dwarves, but were interrupting everyone else constantly for days. Finally got the buggers though but it was awkward. If the beasts were more aggressive, it would have been more dangerous, but perhaps less frustrating. The issue was they are not hostile, but interrupting dwarves as if they are. If they had been hostile, I would have burrowed the citizens, and axes would have found easier purchase when they came down towards us.
Anyway, I will take it under advisement to create archer towers with the fortifications and statues, and not have miners and woodcutters in the military unless they use that same weapon... for now beggers can't be choosers, I'm still early in this fort (for me anyway, maybe someone better could be farther along by now, not sure, it's only the 2nd year).