Not to mention the reasoning behind the 'tail pipe' sequence in The Undiscovered Country.
Spock: Gas. Gas, Captain! Under normal, impulse power she expends fuel like any other vessel. We call it, "plasma", but whatever the Klingon designation is, it is merely ionized gas.
Uhura: What about all that equipment we're carrying to catalogue gaseous anomalies?
*Kirk's brows go up*
Uhura: Well, the thing's gotta have a tailpipe.
If it could work for today's heat seeking missiles, it could certainly work for 23rd Century photon torpedoes.