Well, I think we need all units to have a good balance between usefulness and cost.
Currently there are some rare moments where recruiting a bunch of slingers make sense, but frankly how often does this occur ? This means that in 99% of case slingers are a no-go, there's not much strategy involved...
Sure also, "buffed" slingers should not be made equal to crossbowmen, but there should be more often a choice to do, for example "10 crossbowmen or 25 slingers ?" Here lies strategy
