That point has come up quite a few times. IMO the more you can liberate yourself from numerical measuring of bits of wood, the better the work proceeds. Sometimes it is of course handy to have a number e.g. if for some reason you can't use one piece of wood to mark another and then, as you point out, the System used doesn't matter.
I think that measured plans should only be taken as rough guides e.g. if something is specified at 125 x 63 x 67 mm (unlikely but it could reflect the writer measuring what he has ended up with), then it makes sense to be able to say, "OK, roughly 4 foot by 2 foot by 2 foot". Then you might want to measure your first rough cut of the pieces to 4' 1" but really after the cut to precise length (whatever you may finally settle on e.g. to fit a space) the rest of the measuring is probably a matter for dividers as you are going to be looking for proportionally harmonic bits of wood e.g. 1/2, 1/8, 1/16 of the inital roughly 4' length.