[tartoran]

I agree with you, pain is a useful signal or it could be a systemic damage that wrongly sends these signals. However, when we, humans, refer to pain we clump all the unpleasant things we experience together. One such pain is the emotional /psychological pain and this type of pain is more likely to occur in such a system without a body. This is also a signal/state and is also useful to delineate it from another state. If such system is to experience joy it better know what pain is or otherwise wouldn't be able to delineate.