From Diablo Wiki
Unsummon was most often used by the Necromancer, especially in early versions of Diablo 2, when it was not possible to walk through your own (or anyone else's) minions and a crowd of Skeletons or Revives would entirely block any narrow passageway. The other characters came to use Unsummon as well, once various minion-summoning items were added in D2X and later patches.
Unsummon has not yet been seen in Diablo 3, and it's possible the game will not include such a feature.
- See the page in the Diablo 2 wiki for more details.