I agree that the sail material can take the full load - but downhaul is the greatest threat to sail shape and the life of the sail (according to Skip Elliot). From observation, applying downhaul does stretch the sail but it also bends the mast - both reactions create a depowering effect. If you do not have the support of the bolt rope in the sail, it is reasonable to expect that you will not be able to bend the mast as much with downhaul (since now the sail is stretching more easily) and therefore will not be able to depower as much or at worst, will be applying much more stretch to the sail to reach the same level of depowering. I concede that a bolt rope will shrink as it gets older...but I think it is more of a springy effect in that they shrink but will stretch back to original length with minimal applied stress. That's my opinion for what it's worth.