I would be interested to know how you use them in war, I find them useless. A light horse can do almost everything a skirmisher does but better.
I create a scout at the start. I do not slot the scout so card unless I am Incan, German or want to play with skirmishers (which sometimes I do)
This scout first role is not to scout but to protect my base from enemy scouts as early barb attacks are game changing more than a +2 culture CS. Any chance I get to attack a barb scout or kill a spearman I will take as these soon rack up to 15 points. Even if I get to 10 points a goody hut will make that 15 but primarily garbing to promotion 1. I do not baby my scout and often use it to ZOC a city or sacrifice but quite a lot of games it survives this first phase.
Getting the second promotion is the real hard one (need 30 XP) but as each wonder is 10 XP it can be done and any war you get into can also help because normal XP counts and taking a city is a minimum of 10 XP. If I am Germany or Inca perhaps 20XP. Once again, I do not typically expect survival but if I get to level 2 I am now interested in my unit.
If it is still a scout it typically follows standard scout duties because it is weak melee of support, flank, ZOC, chase, bait and city capture but perhaps at this stage I will mother it a bit because when it becomes a skirmisher the world changes.
At skirmisher it become ranged and a ranged move after attacking unit is hard to kill, especially if it can go over hills or through trees fast. Your skirmisher is a prime target for the AI because it has a weak combat strength so move it to a hill 1 tile away from a warrior/sword whatever and let the unit come adjacent to them and then shoot and backup a tile, repeat if possible. This approach is best done at the side or rear of an enemy civ, you would be amazed how the enemy civ will send troops to stop it, if they do not, pillage or just shoot cities without walls for XP. The idea is to get it to level3 but misdirection of enemy troops and pillaging are both powerful for it.
If you have not had a level 3 skirmisher, you really need to play with one once. There is a windows of maybe 20-30 turns when it can own because it is a melee strength 40, range strength 50 move after attacking unit and with a general becomes a serious nuisance. This 3rd promotion +20 in all situations also makes it valuable moving up promotions.
One very subtle difference to a practiced player is the skirmisher is targeted as if it is a strength 20 unit even though it is strength 40. Using this is useful.
Typically it will die sometime but it will be a game I really enjoyed because it was another cool unit to play with. I use em most games but try not to mother them because I feel that is a mistake but when they come good it just adds a new dimension to an attack.