Where they work well is when there is some disconnect between the product (and the product owner) and the team. If that isn't a problem you have (which is brilliant if not) then this should mean one SM can support multiple teams, as there isn't a lot them to do.
They should also be there to take some management effort from the team, and ensure that high quality stories are coming in, that the team are effectively communicating, and that changes to process are made (where they can add value for the team). The ultimate goal for a good SM, is that the team does not need them, and that they can step away.
I believe Scrum can be useful for teams - but scrum/agile is not a hammer to hit all teams with. You need to know/communicate how you think it will improve the team, how you will measure it and most importantly you need to bring the team along with you on the journey. It sounds like that might be an issue here?