Depends what the thing is. General guideline, the more scientific, and ultimately mathematical, a thing is, the less opinion matters. So it's not a silly argument at all. Medicine is relatively scientific, so opinions start becoming less significant if there is no scientific evidence to support them.
This opinion (JW and transfusions) has no scientific evidence to support it, therefore it should be afforded at the very least far less weight than the contrary, if not no weight at all.