I'm not sure if we really need the cannot_skirmish.

The main purpose of cannot_skirmish is for the AI. Since it doesn't know when to turn on or off the skirmishing mode by itself. However, the javelinmen units can have the "prec" which is much better for non-skirmishing types of units which have javs as primary weapon. The others which have arrows and slings will probably be better with skirmishing ability to optimize their longer range.

However, the Archer-Spearman is good example of a situation where we may adopt the cannot_skirmish attribute because we don't want the AI to have the Archer-Spearman skirmishing against cavalry.