Slingers are essential to the deck. They are so essential that you should never discard them for magic, just summon them SOMEWHERE, you want all four Slingers played from deck, each and every game.
Their extra attacks allow Cave Goblins to generate more damage per turn than their opponents on a consistent basis.
However, using them is not easy. They are very easy to kill at 1 life and many decks have tricks to kill them without ever attacking (like direct damage).
How to use Slingers properly and keep them alive is one of the main skills to master when playing as Cave Goblins.
Keep them at range most of the time, Sly them away and even have them attack non-optimal targets rather than expose them early to a quick death. A Slinger slowly chipping away at an enemy gate can be of great value.