Yup .....this is the most valid reason for mast head lock on a carbon rig. Its not the mast compression, it is because the distance between head of the sail and the cleat can change and so halyard tension can change. Merely adding vang on a bendy rig (good in big breeze) also eases the halyard (bad in big breeze) so mast head lock will effectively eliminate this stretch.The definition of mast compression decides whether you think the compression changes. I agree that halyard cleat location does not effect compression at the bast of the mast - that is provided by mainsail forces, standing rigging (not applicable here) etc. However, the mast will be compressed between the head of the sail and the location of the cleat by the halyard tension and any cunningham tension applied to the sail. Having a halyard lock, or cleat at the top, ensures that this compression is only experienced by the very top of the mast, which is the definition being applied here.There is less halyard stretch, although most dinghy sail designs have a softer luff than the stretch in a typical modern line used as a halyard so I expect it is imperceptible in a dinghy of this size with a dacron sail.When the halyard cleat is at the top the mast there is hardly any halyard to stretch so the Cunningham works more effectively. Also, the mast is only under half compression compared to a base cleat when the Cunningham is yanked on. Don't know how important that is in boats like the Aero though.
The mast compression is the same whether you locate the halyard cleat at the bottom of the mast, half way up the mast or at the top of the mast. As one naval architect explained to us......."Hold up a broomstick in your left hand with a 40lb weight tied to a line going through a sheeve at the top of the broom stick.....if you tie the line off by the sheeve or at the bottom of the broomstick, the compression is still 40lbs.......compression is affected by the tension on the main and the equal and opposing force provided by the mast, not where you tie off the halyard"
Another trick to prove this......apply max luff tension with the halyard and the mast lock off.....so the mast is pre-bent under compression. Lock the mast lock so the halyard is locked... release the halyard. The mast will not spring back up to vertical under less compression.
Old wives/Etchell sailors lore.
With an Aero, the halyard cleat is external to the mast, so with the amount that the rig bends it must cleat at the top. Otherwise, the distance between mast tip and where a mast base cleat would be would shorten too much as the rig bends, reducing halyard tension and allowing the sail to sag. If the halyard tension is kept high enough to ensure a tight luff when the rig bends with a mast base cleat, you would, as Amati states, simply create a bow.