This is a great point. It's a very sci-fi term - from Invasion of the Body Snatchers to Aliens, where the "alien" life takes advantage of an existing organism. Also make me think of viral epidemics. Hrm.
Right, the term hosts makes way more sense in that context. Host is not a great term for describing the robots in the park. A large proportion are downright aggressive to guests. A more appropriate term, given the idea of being involved in a narrative, would be something akin to Disney's cast members.
Going back to my own point about erasing a sentient host to make room for a human conciousness being at the heart of the disagreement between Ford and Arnold here are two relevant comments:
From Some Pig No Doubt above we have "Ford's view that Arnold took a dim view of humanity, preferring hosts over humans"
And Ford's comment that "You can't play God without being acquianted with the Devil." Where playing God could easily be construed as giving humans longer / indefinite lives and being acquainted with the Devil could be the "murder" of sentient hosts.