wiggy, thank you. In my view as a (now retired) captain, they were worth their weight in gold when the decision to divert or carry on was made. When over the Atlantic or Pacific, their expertise (contacted by satcom) permitted more decisions to proceed than divert but gave us the confidence that we needed to divert when unsure. The airline usually covers the costs, which can be substantial if hotels, missed connections, etc are included in the operation.
Off-loading a passenger due purely for non-medically-derived behaviour (drunk, abusive, threatening, etc) has typically been "at their cost" and not the cost of the airline. Also, there is the criminal-record issue for interference with the flight as well, which in my view, is, along with costs, fully deserved.