There's no easy answer, but in general, the only people who oppose this sort of action are the civil libertarians, and those who want to do the wrong thing on board.
Personally, I don't have a problem, and it would even give me some confidence when paxing to have a John Wayne or two on board. I think psychological testing would be a good idea in the selection process.
As a Captain, I have been involved in a number of disruptive pax incidents, and it is uncomfortable for everyone, and on each occasion, has consumed a huge amount of our time. It's disruptive to our duties, and therefore a potential safety problem.
I say that the advantages of having a properly trained and equipped air marshall available to deal with these incidents is a move in the right direction. Just the deterrent factor is worth considering.