It is very hard to make a lot of use out of the Skirmisher, since this is a Recon unit and exploring is no longer a priority when late Medieval rolls around. Not an effective combat unit as well, it is surely more durable than the Scout, but it has melee Combat Strength of the basic Warrior and its Ranged Strength is only slightly higher than the Archer, it does not deal well with Medieval Barbarians and it is too late to run Survey policy card to amp up its XP gain. Thus, unless you play as the Incans with a unique Skirmisher (Warak'aq) or as the Scottish (try to earn XP to earn the Ambush promotion for the Highlander using Survey policy card), you can just save your gold/production and ignore this unit and your empire still does not suffer too much. Until Spec Ops is unlocked, the main purpose of Recon units is still exploring safely, and your Scout still fulfills this purpose without any extra investment.
Civilopedia entry Edit
Encounters between small groups of soldiers are generally termed skirmishes. These are too small to warrant the definition of battle, and are characterized by low numbers of combatants on both sides. Skirmishes may precede a general battle, as reconnaissance edges into recon-by-force. In these situations, soldiers who are most effective fighting in small groups while remaining in contact with the larger army will tend to triumph.
As the late classical age edged into the medieval period, there was a general decrease in the size of fighting forces, in part because state-sponsored armies were less common, but also because of the rise of the personal retinue by the nobility as part of the feudal system. Viking nobles had personal households that accompanied them on raids. As the Hundred Years' War ground on, English soldiers practiced the chevauchee—a destructive pattern of raiding by small groups of soldiers (often without sanction). The skirmisher represents these kinds of forces. Tough, seasoned warriors, capable of living in the field, surviving on their wits, and ready to withdraw if faced by tougher foes.