Yes they are but for a specific purpose e.g. a backstay to support a full height spinnaker or runners to support an inner staysail or storm jib. Or in the case of non B&R rigs to create the fourth angle of support.Permanent and running BS on fractional boats are very common. Without getting into the rotating mast and the loading of individual side stays, if I or anyone else wanted to add one what is it to anyone else. Why would it have to be braided cable, why not Dyneema, hull why not yellow braided nylon? If it doesn't do anything then why should anyone else care?
You are correct it is nothing to with anyone else if you do or not do this but this forum is used by lots of inexperienced sailors and to leave them with the impression that simply installing a backstay on the standard rig will allow them to run downwind with more sail than prudent in strong winds is dangerous. It does not do anything to save the rig but it can cause over confidence in an inexperienced sailor or worse apply additional stresses by tensioning the top part of the unsupported mast.
Dyneema would be good any non stretch line would be good, nylon not so good as it can stretch leading to pumping of the mast.
The point which was trying to made is unless you have modified the rig to carry additional sails a backstay is not required and does nothing to protect or save the rig which is what the post was about. Backstays have a place but the inexperienced sailor should learn what they are used for and learn how a B&R rig is designed not just assume that by adding one they have somehow strengthened the rig, because if not used correctly the opposite is true.
Just cause they don't have one like most other rigs don't mean they need one.
