That is a clear cut case of my reason # 3) "They have an exceptionally strong vang with a lift spring and an extremely strong boom and gooseneck, so the mainsheet is not used to control twist at all." In the case of the Xp55 it appears to be a carbon fiber boom. I can only speculate on what the gooseneck and mast reinforcing must be with that kind of thrust.
But I would disagree that you can control the leech as efficiently with the vang as with the mainsheet and traveler. Vangs tend to be pretty blunt instruments that are hard to control with the kind of precision that comes from pairing the traveler and mainsheet.
No disrespect my sailing brother but seeing that on my 74 force 5 bought new my and my 84 j/30 again bought new both with just the block and line vangs using it to set the leech I never have had a failure on either at the points you refer. On my 01 j/32 I have had to replace the bushing on the upper part of the goose neck between the boom and mast due to wear but still after somewhere around 15 years no failure on that one either.
Personally that was the way I was trained called "vang sheeting" when you head out using the sheet adjusting the leech to the conditions once set take the slack out of the vang then the rest of the day use either the sheet or traveler to open and close the door in the puffs. All the other sail depowering done with the other adjustments outhaul, backstay, cuningham etc. Ever watch upper level Finn and Laser sailors all of them sail the same way.
Now I've only been sailing since 1964 I know my experience is limited but never had failure on any of those points. Hell on the 30 we use to do what we'd call a spinning take down. Thats when flying the spinnaker in a 30 knot breeze you broach the boat and when it spins into the wind you blow the halyard and the sail just drops to the deck. I don't know how many times the boom would be stuck in the drink no goose neck/vang failure.
Over the 50 some years I have been sailing soooooooo many times I've heard stories of people busting out the goose neck on a gybe but have never meet the person it's happen to, always a friend of a friend. I have to tell ya I think its urban legend and anybody that, that has actually happened to I'd like to talk to, because I too would like to be able to say"I know this guy".
When you really look at the physics of a boom there are a few other points where you should have failures like .25 d shackles bails traveler cars all fail points of sudden impact on a gybe that have much greater leverage than the goose neck end of the boom. Those are all the points I have had failures, couple of times I have had those little ball bearings from a Harken windward sheeting car exploding into the cockpit I've had d shackles blocks bails all just fail in breezy conditions never a vang or goose neck.
I just wanted to put it out there for the people with that mythological fear that they are going to bust out their mast to let them know it's a common practice sailors use. The efficiency is you are making one adjustment instead of 2 (vang verses sheet and traveler) in the puffs dump off air by dropping the boom not twisted it off the top and then try to get that setting back when you can just bring the boom back up. Don't over think it adjusting the leech all you're doing is alining the top of the sail with the bottom maybe a hair of a twist. That can be done at the beginning of the day with vang and forgot. Angle of attack you do with either the sheet or traveler all the other depowering is done by other adjustments. If you feel the need to be constantly inching in and out on the sheet and traveler looking for the "philosopher stone" thats not trimming it's OCD and this forum is the wrong one.
But hey if it makes you feel better be my guest...
Oh yeah the 55 I raced on had an aluminum boom and mast carbon was a extra ;-)