The point is they can fire and do damage while stationary. Instead of charging in and losing hitpoints. IMO its quite cool, and more realistic. I would like to have archers also this way, because its not realistic having long range on archers like cannons and siege weapons.
I hope its possible to mod it, making them able to defend a little better as tradeoff and decrease range to 1. Archers were not bad for defending a position in the old days, and in previous civ games they were main defense unit for cities.
Crossbows realistically also have much shorter range, so it should be the same for it.
Gatling guns would then be a replacement for outdated crossbowmen in that case.
So IMO all these should have some defense capability for holding a position on the map (like gatling gun currently have) but not too high because of game balance.
Someone who modded this already?