they dont work.

plasma is just really hot gas- it rises

plenty of things more practical

a plasma is when individual atoms have been energized to the point at which many, most or all of their electrons have detached and wander freely between the various nuclei. 

the range of plasma weapons is going to be quite short and it's likely they will not be all that useful for space combat.  they may be handy for ground or air combat but that would call for some serious breakthrough technologies.

Yeah, but they would be really, really, really short range. If you want to destroy bunkers you'd be better off with thermobaric grenades. If you just want to burn stuff you'd be better off with napalm. If napalm isn't isn't hot enough or sci-fi enough, you could invoke napalm with suspended thermite or something.