In cases where I can't possibly know about another person's capabilities, I'd rather believe that they can change, make decisions, think rationally, respond rather than react, and so on. I wouldn't go so far as to say that they deliberately choose not to exercise those capabilities in the overwhelming number of instances where people do not do those things, but I'd feel pretty hopeless if I assumed that they couldn't. Also, it seems kind of elitist and disrespectful to assume that they can't.