There are a number of advantages to both sallies and skirmish skinks that make them excellent value for their points due to lethality, mobility and survivability (OK, not so much for T2 skinks

). Here is quick top 3 for each:
Sallies:
1) Skirmish - great mobility, -1 to shoot at, stubborn in the woods and they can march & shoot. The latter part means they effectively have a range of up to 30" (12" march + 8" template + arty die roll up to 10").
2) Monster & Handler rules - this allows the handlers to be ignored almost completely, meaning the enemy is always fighting against the sallie toughness, WS and AS. Even if a wound is taken, it is randomized and on a 5+ kills a handler instead of wounding a sallie - essentially a 5+ ward save vs cannon balls! In addition all of the handler attacks can be directed against any model in base contact with a sallie but the handlers cannot be attacked directly. This allows a pile of attacks to be directed against enemy wizards or other soft targets.
3) Flame shooting attack - unlike a breath weapon, it can be used every turn. As others have noted, it is devastating against hordes.
Skirmish skinks:
1) Skirmish - as for sallies, giving them a 24" range effectively for shooting.
2) Poison - autowound on big monster or just help grind down an enemy unit - even chaos knights will roll 1s on their AS if they have to do enough of them.
3) disposable martyrs - a unit is cheap enough that its loss is no big deal. Since charging units now align to the skirmisher facing, this makes the unit ideal for deflecting enemy units.