After 20 years of sailing cats I know absolutely nothing about spin flying.

So I have a question for all you spinnaker guys. While sailing aboard my buddy's mystere 5.5 he cranks in the main sheet while downwind then sets the chute, and keeps the main sheeted. He claims this is because the boat doesn't have a backstay and without a sheeted main we would demast.

My question is, the chain plates for the shrouds are aft of the forward beam...aren't they strong enough to hold the mast up by themselves?

For deep downwind sailing it makes more sense to me to ease way off on the main even with the spinnaker up.