One plasma command squad is a great complement for a special weapon squad on a Chimera/Valkyrie, where you will also want to have 1-2 characters.
This way, they get expensive soon, however. The other useful setup I've tried is as 4x snipers, camping on your backlines. It protects against deepstrikes, is dirty cheap, can be useful and sometimes even dangerous, and you won't cry if you lose it. Your opponent will not always have the firepower to spare to remove it, and often will leave your snipers there. 3x of these squads is a mere handful of pts, although you will need to have 3x Officers. They perform best as Cadians, of course, but you will never issue an order to them.
Sadly, very sadly, their most obvious and fluffy setup, as a support squad with vox, medic, and banner, is vastly underperforming, overpriced, and generally useless.