Perhaps I used the word “preventer” too loosely as it should possibly be reserved for the end of boom system of boom control which takes the drag of the boom tip if it rolls into the sea when running hard and “prevents” if you will, the boom from breaking and also prevents the boom from sweeping the deck dangerously to both boat and crew in an uncontrolled gybe.
The side deck arrangement I described should perhaps be better described as a “stabilizer” for the boom, especially when the boom end is still over the deck where it often has latitude to “jump” around. The end of boom preventer system is only effective when the boom end is well away from the hull and the preventer gets a good angle to the boom end.
My side decks system’s primary purpose is as a vang: secondarily as a stabilizer. And you are correct to flag concern about boom damage. I have taken a number of precautions to make boom damage unlikely.
To get back to the initial question: boom brakes are, to my observation, not preventers (they are boom movement “slowers”) and present a similar, though lesser, concern, I would think, about boom damage. They make gybes more controlled and thereby safer for boat and crew. But I suspect that an end of boom preventer system for those with boom brakes would be wise for those times when a boom tip might be dipped into the sea.
Lastly, that is a fine way to make the "pennant" of the end boom preventer always available to be attached to the longer line going forward to the bow and back to the cockpit.
My thoughts, Dick Stevenson, s/v Alchemy