Speaking as one who has designed rigs I can suggest only that there are really only 2 reasons for a multiple-spreader rig. The first is because the spar has numerous stress points by design-- a fractional-rig headstay or intermediate stays, such as a Hunter 37. The second is because a disproportionately lightweight, overly tall or undersized spar is being used to save weight aloft. This seems ironic since the clutter of more rigging aloft would add to weight; but with many highly-technical designers it's just where the weight is that matters, and they manage the distribution of the weight they do have.
On a cruising boats with masthead rigs I don't see any reason to have multiple spreaders, providing the width of the rig's base (at the deck) is wide enough to provide ample mechanical advantage for the stays to have effect in stiffening the mast.
And you're right, Russf-- it adds plenty of complication. I have tuned enough C44 rigs to know that!
