I went through this issue when I was new to the game, back when it was released.

It's a very frustrating aspect of how Aetherial gear works.
It depends on the stats it is randomised with.

If it has random stats more akin to tank gear (Parry for example), then a Dragoon cannot need it.

I had complaints then, and I still have complaints.
Scale Mail is DRG gear, we should be able to need it.