If you were aboard his boat and in line with that preventer line and got hit by the block when the line let go, you'd likely be dead or headed for the hospital. Hoping something will break at the right moment in time is rather foolish and stupid IMHO.
Before your remarks could be considered valid you really ought to know the size and location of the block and the way in which it was deliberately weakened to ensure failure under the loads he tested it under. Then again the way it was rigged and what would happen when it failed is relevant too.
Clearly the boom snapping over was a serious health hazard, but rather that than loosing the yacht.
Also please understand your opinion is being weighed against the Chief Surveyor for the British Navy, albeit of some 40 years ago.
The block was about 1" long in total and, as previously stated made of an aluminium alloy with a sheave diameter of little more than 1/2".
From memory the preventer line came from the bow, through the weakened block at the boom end, along the boom, through a second conventional block on deck at the foot of the mast and thence back to the cockpit. Should the block fail the sheave would have been thrown forward.
And then again, cotton or hemp ropes did not contain the same strain energy that today's synthetics do. We realised that it was better to have the block fail than to have the preventer break and face its unknown whiplash effect.
And Frank was a Scotsman and didn't want to pay for a new length of rope when a shilling Woolworths block would do!
One notes in passing that it is quite normal for engineers to design links which are designed to break and shear pins are used universally and even in outboard motor transmissions. Quill shafts are routinely specified in aircraft gas turbine engines with necks specifically designed to fail at a predetermined torque should an accessory seize.
They have even been used on catamaran rigs to cause the rig to fall down and save lives rather than have the vessel turn upside down.
Unfortunately my friend's 30' cruising cat was not so equipped and his wife drowned when it capsized in the Solent. The mainsheet jammed on the winch and the cat capsized before anyone could get to it with a knife. A version of old Frank's weakened block on the mainsheet or shearpins on the shrouds could have saved her.
I was racing the yacht with the preventer at the time, also in the Solent.
I have friends who know nothing about sailing but call my "boat" a yacht. Of course, in Oklahoma if its bigger than a jon boat it might qualify as a yacht. Imho, you can call it what you want. I prefer to call it a sailboat however. Calling it a yacht does seem a little high falutin to me. Sooo... I guess I agree with you but not in principle? 
