I too have not built many barracks and the like because I just don't feel the benefits justify the cost. But unlike some people here, I don't see why that's a problem. The option is there to build it or not.
If you want to get the most of these buildings, the obvious strategy is to specialize your cities. One builds land units, the other mounted, the other naval. One building to improve those units in each city.
If you want to get the most of these buildings, the obvious strategy is to specialize your cities. One builds land units, the other mounted, the other naval. One building to improve those units in each city.