I realised that you meant to add rather than swap, but it doesn't change the thrust of my argument, which is that I'm not persuaded that adding a boombrake adds anything that a suitably set up vang-preventer system cannot already provide.
In the sentence you quote, in which I use the word 'swap', I'm suggesting a swap happening in the mind, rather than on the boat.
In other words, it's an abstraction to illustrate that both systems are essentially the same animal, so it seems to me to apply equally to the argument against duplication as it does to the one against substitution.
I'm open to persuasion, though, if there's something you can point out that a boombrake provides which is not covered by the tail-management variations I mention above in relation to preventer/vangs.
Having said that, the persuasion would face a bit of a head wind
, because my natural inclination is to optimise one system to do everything needed rather than provide two overlapping and more specialised systems - each of which will (unless the person has unlimited time) have to share the effort and resources required to understand it and refine it.
On a different but related tack, I was privately a bit perplexed when someone further up this thread suggested fitting two preventers on each side, with the intention of using them simultaneously.
To me this is a bit like telling two people to do the same job. Often you get the situation where neither will do a particularly good job, because they think the other will take up the slack.
More specifically to this thread, it means preventer operations will take twice as much fiddling - I'm not sure that's conducive to reliability
of outcome, especially when the chips are down.
I suppose I'm also mindful of how many ropes there already are in most cockpits, and that their lustful proclivities to intertwine at the slightest opportunity are exponentially related to their number and length.
Some boombrakes (and most of the better ones) also have lines coming back to the cockpit
, so I suppose this reservation applies, in those cases, to your suggestion.