The armor is pretty. It's "different". I think that's all there is to it.

If it wouldn't use any additional unit slot and just a different texture, even if it had no different stats or whatever, yes it would be neat indeed to have them recruitable in Dacia.

Honestly, I raise my Seleucid armies by visual. Some units look great and I know I have a good army when the mix and match looks about right.

Just kidding, you know, but the Seleucids do have some really nice and cool looking, "unique" or "different" units.