If the boat has standard spreaders, versus swept back spreaders, and the shrouds connect at the same point on the mast as the forestay, the mast is supported at that point in THREE directions, but not the fourth. It can pump—which can lead to the mast fatiguing and failing. I didn't say that it was always the case, which is why I said IT MAY NOT BE SUPPORTED. It really depends on the individual boat in question.
As Tartan34C has pointed out... the use of running backstays, jumper stays or swept spreaders can offset this risk...but without one of the above, it is certainly a possible risk.
Hokay.. I see where the mis-understanding is. On the last two boats I've owned (both fractional rigs with diamonds), the shrouds do all connect at the same point on the mast, but the mast IS
correctly supported in THREE directions by the stays being led to chainplates AFT
of the mast - forming a 3-point star on deck.
Granted, this set-up limits how far you can let the boom out downwind (but hey, a dead run is slow anyway) and the mast would be far better supported by side stays+backstay (4 points, a bit more complicated), but where does the "pumping" come from??
I'm thinking of changing the rig to swept spreaders, backstay + lowers, so I'd like to understand how this works..
Thanks for your patience