c elegans has very primitive, simple behaviours. It's not really possible to get something useful out of it about either our cognitive functions or our brain disorders. The things that regard single cell pathologies (e.g. plasticity) are already studied in vitro in mammalian cells. There s probably many cognitive phenomena that only become apparent in large brain sizes, so i m not sure this method scales up.
In what situation WOULD you be able to expect to extract "something useful" about "our cognitive fucntions or our brain disorders"? It seems silly to think we could learn anything about such complex things without understanding something simpler first, hence the approach of validating models of simpler structures.