Remember that the mast does not have any integrated support above the hound where the stays attach. The higher up you place your spinnaker head, the more bending force you apply to the mast. The lower you attach your spinnaker (Bill Roberts recommended up 1/3 the distance from the hound to the tip of the mast maximum for goof proof sailing) the less chance you have of inverting or breaking the mast. The higher you go, the more the mast depends on mainsheet tension to support it. Monohulls that run masthead spinnakers use backstays and/or checkstays to support the mast to be able to run spinnakers as high as possible - we only have mainsheet tension through the sail to support up high.