Not quite sure why personal goals or motivation has anything to do with this..
it has because if, let's say, i'm a runner and more specifically i'm motivated by marathons, i don't give a fuck about fast twitch fibers, and that ends the discussion.
(just to make a gross generalization. but my point was more about your hamletic doubt between two campus board routines, than about the non-reversibility issue).
whether and how the non-reversibility thing is important in climbing is an interesting issue in itself.
(edit : apparently it's not such a big issue anyway, if learner is right?
My 2 cents is that it's too early to know for sure, and we're still in an era where empiricism works best.
An answer sounding like "doubles improve your reach and hand-eye coordination in big dynamic moves" should satisfy you.
Still too early to focus on the science behind it, imho.
remember that most sports physiology studies are about "simple" tasks such as running or weightlifting, and we have a hard time applying knoledge gained in those areas to climbing...
maybe in 10 years climbing comps will go olympic and all WC athletes in boulder, lead and speed competitions will be followed by a medical staff à-la-Patxi...
then we'll have the kind of answers you are looking for, and knowing those answer will make a true difference (at the elite level).
(hint : if we only look at muscular issues, 90% of the times the limiting factor on any climb/move comes from your finger flexors being too weak or not having enough stamina. Empirically speaking, you have to hold those crimps, pockets, slopers and pinches!
Now, your finger flexors normally perform a series of isometric contractions during a climb.
Whereas those sports or exercises from which we try to borrow our knowledge mostly involve bigger muscles and in a much more dynamic fashion...
that's the root of the problem and the reason why i'm skeptical.
Though, if anyone has good scientific resources about the physiology of intermittent isometric contractions in a smaller muscle group, i'm very interested...)