Since plasma is really just super-heated gas, it will rapidly disperse unless somehow contained. Any civilization with the technology to keep plasma contained while propelling it over any significant distance (especially through atmosphere) would have far better ways to damage enemies than by throwing blobs of hot gas at them.
Examples of Plasma Weapons
- PPGs in Babylon 5
- Imperial plasma guns and Tau pulse weapons in Warhammer 40,000
- Romulan plasma torpedoes in Star Trek
- Asgard plasma beam weapons in Stargate
- Mike Wong's Plasma Weapons essay points out the problems with plasma weapons as a science fiction concept.