However, the role and attitude of the author seems ultimately helpful to his patients. If he can screen the suffers of diagnosable and treatable mental illnesses from the hard to explain cases, he can get proper treatment for the former group. By not denying the existence of demonic possession, he remains in the good graces of religious leaders, who are likely to bring him patients who otherwise would receive no psychiatric evaluation.
However, leaving the later group in the hands of exorcists is questionably moral. But if honestly, as he claims, there are cases where "the symptoms in question have no conceivable medical cause" then I guess there is little else he can do.
Of course, writing a click-bait article about diagnosing demonic possession is unprofessional of both Gallagher and the Washington Post.