Think of them more like the Kung-Fu monks archetype. Incredible power from insight and meditation, but insular and detached. Occasionally comes out to raise awareness about their religion and trade for little things. For them to work thematically, they must be mysterious, powerful, and withdrawn. I intend that those with high Willpower can resist their powers (needs testing). The basic idea is that for one to become an Adherent of the Mind, they must cease to care much about physical world, eventually all but abandoning it (hence the lack of bodies).
I'd intended that the main reason for trading with them would be some nifty cloth they make by telekinetically arranging molecules, but the don't seem to produce it. With Putnam's update, their useful goods will be flax, logs, and hive products.
Right now there's two things (that I know of) that they're vulnerable to:
-Occulentibus
-Each other
-Plus the usual things (crushing, freezing, encasing in obsidian).
Bear in mind that when I introduced them, I said there's currently no convenient way to kill them. I wanted to avoid some "hit with silver bullet" method, so I'd intended to allow Adventurers to learn mind powers and be able to slay them in a mental battle. At some point there should be other creatures who possess mental powers, perhaps not as broad as those of beings of pure thought, but potent enough to allow them to do non-physical battle. I think it would make sense for Formic Queens to have innate Psionic ability, what with the whole hive mind thing.
PS: I have never, EVER seen anyone but the player actually pick up an item in adventure mode, and never seen non-player units in fort mode pick up weapons lying around, so I'm still skeptical of the claim that they're grabbing spears. It's quite odd and puzzling.
Also, try pitting them against Treelords. I've never seen that fight end.