Consider Watson as a research assistant on a medical diagnostic team. A patient comes in with a puzzling set of symptoms. Watson launches a search through hundreds of thousands of journal articles and case studies. It returns with six possible diagnoses and its level of confidence in each one -- along with links to the evidence it studied. Let's say two of those six are far-fetched... [d]octors know enough to rule out a few others. Still, if even one of those six possibilities leads the team toward plausible answers they hadn't considered, the machine will have done its job.
I'm sure that's the rationale the maniacal cyborgs use in some sci-fi movie, too, back when they're first verging on sentience. But until the replicants destroy us all, it does sound pretty cool.
Bob Harris has written books, American TV dramas, and Mexican telenovelas; circled the world 3 times; lost 5 games of Jeopardy (and won a few, too); and dabbled eagerly.