When you start playing a war game, you have to create a good strategy and army lineups to win the battle and mostly some players choose archers because of its range.
Anyway, back on topic. I think Castamir´s army would still think or want to believe they fight for Gondor - after all, it was more of a conservatives rising anyway - so I suppose they would still wear Minas Tirith uniforms, possibly with signs of their hearkening back to the old days - Numenorean helmets, the more stylised "Second Age" tree of Gondor on their shields, and such.
"... Telchar wrought it in the deeps of time."
-On Andùril, The Lord of the Rings