That's not what I meant. They're not controllable, or at least they're not meant to be. You could always build them in to your own vehicle but I certainly wouldn't.
What I'm saying is that you aren't going to be driving around in one.
Well no, not really. I've already concidered building a map with dispgen and then adding brushvehicles. The problem, however, is that brush vehicles suck for performance, and I'm also not sure all "guns" will work.
So when I get in to modelling I might do some fighting vehicles and "fake" their death. You place a brush around your vehicle, that brush takes 'damage'. At certain points your vehicle activates some env_fire's attached to your vehicle at different spots. And finally an explosion and a -kill output to your model vehicle.