Indeed. My memory is failing me but I seem to recall having heard or read someone talking about the brain and stating that its sole purpose is to move the body. Thus, an organism that does not move does not need a brain, and as a proof of this idea he mentioned that some animals, like some medusa, loose their brain once they fix themselves to the floor (turning into polyps in the case of medusa).
Daniel Wolpert has a TED talk where he makes this point [1]. And I think he's basically got it right. Certainly there are organisms that can move without a nervous system (single celled amoebas, venus fly traps, etc) but by and large you need an (electrical) brain if you are going to react to something on a fast timescale.
