I believe Watson, in practical application, would help solve the human problem of "digging up" the information necessary to produce the best diagnosis and treatments possible. Several years of medical experience, training and keeping up to date with medical sciences is necessary to actually make the best possible decisions. As a result, there's a huge amount of information from many facets of medicine for doctors to parse, much of which can be outside their area of expertise... specialty areas, the latest on break outs, internal medicine, statistics, demographics, new illnesses and so on... This kind of crunching sounds like something Watson can help with. If it can, it could greatly assist doctors of all kinds.
However, Watson will not replace (competent) humans in the actual decision making process. Ultimately, we are far, far, far away from making a question and answer machine that will usurp human judgement (even with all of its flaws). It could offer a valuable second opinion, though... and at the very least could make a great "research assistant".