Seriously though. I have only had my boat six or so months but I had though about running backstay for when you are going down wind in anything over twenty. And even then it would be more for piece of mind and to ease strain on the boat and not because I actually thought the mast would fail. Note proper rig tension is a must with this type of rig.
Roger has cleverly designed this boat so that she physically can't stand up to forces that would endanger the rig.
From what I have seen the backstay add on is low on the list of changes people make to these boats.
Ixneigh
You will find delevi sailed in heavy winds in San Francisco Bay in his with upgraded sails - I don't remember if Leon added a backstay or not, but certainly that would be a good search start...
I've never heard of any kind of failure on a Mac that a back-stay would have resolved. The spreaders and after side stays do a perfectly good job of tension the mast in balance with the fore-stay. It's very low on my list of things to worry about.
I wouldnt worry to much about what your Proffesional sailer mates told ya Kurtz. They can't be too proffesional anyway, because if they were, they would know that a boat like this, with this rig, just doesnt need a backstay
One reason for a backstay is to induce mast bend; that's why your pro friends criticise the Mac. Perhaps you could point out to them that the mast on an rotates and so bend might be a bit of a problem. I bet they also criticise all those cruising boats that have in-mast furling or any other non-purist feature.
If you pitch the motor over the side, the pro guys will like the boat a lot more. Suddenly they will notice the awesome rotating mast and they will all wonder how maybe they could have one on their boat
I assume the force from a sheeted mainsail takes quite a bit of the force a backstay would. I have running backstays. They help keep the forestay tight at the dock and the furled jib doesn't whip around as much. I also use them when I'm sailing on just a headsail. Having said that, I'm not convinced they're worth the hassle.