How one treats an old bike or frame might differ justly according to his intent. If he has no interest in experiencing an old bicycle, and needs the old frame only to make himself a bike to ride, then his needs are well served by putting current parts on it. However if his interest is in veteran cycling (veteran, rather than vintage, because not all worthy old bikes, such as mass-produced lightweights, fall into the category of excellence that "vintage" should convey), then it makes no sense to put many current parts on old frames. To me it is a matter of the context in which the bike is ridden.
Members of the Veteran-Cycle Club debate this issue from time to time in the letters page of our journal. The Club takes this stance in its rules: 1) there are no date restrictions for bikes used on V-CC rides; 2) members should ride any bike they think would be of interest to other members; and 3) "bitzas" - bits of this, bits of that - have no place on Club runs. Therefore in the context of a veteran cycle ride, originality or period-correct is the goal, whether one rides an ordinary, a pre-WWI cross-frame roadster, a 1950s hand built lightweight, or a Phat Chopper - a Phat Chopper has appeared on a few recent V-CC rides. The point of a V-CC ride is to enjoy interesting period bikes in the outdoors with friends, and the period may be last year - usually not, but sometimes yes.
At the same time, a V-CC ride is a social occasion with friends, and friends don't, in the words of John Pinkerton, "complain about somebody having the wrong tyre tread." Old bikes brought back to the road for veteran cycling often are works in progress: get some signature parts right that give you the authentic experience, even if that means using a crummy old Gran Sport derailleur, start riding it, and look for the final parts over time. One might also need to adapt to local conditions, and if that means using a Suntour derailleur for the hills until that rare French touring derailleur from the Fifties with the necessary range surfaces, and somehow is overlooked by well-heeled collectors, then use the Suntour. After all, if the bike had been ridden continuously for fifty years, that is likely what would be on it; whoever rode my current fixed-gear frame from 1959 into the 1990s last used the original GS front derailleur and a Shimano Alivio rear.
If one is not riding in the context of a group of old-bike enthusiasts, and is not interested in the unique experience and change of pace that an old bike with period equipment provides, then using any parts that he likes makes perfect sense. By the same token, within the context of a veteran cycle ride, it is not elitist or snobbish to aim for period correctness.
What does not make sense to me is putting bikes on the wall and not using them. I have handled the First Folio of Shakespeare, books inscribed to friends by Robert Frost with poems in his distinctive, craggy handwriting, and I have in my care a copy of William Morris's _News from Nowhere_ printed by his own hand at the Kelmscott Press in 1893. I would not take those books on the subway to read, even if that was allowed. How many bicycles are in that class of cultural artifact? Dr. Paul Dudley White's bicycle hangs on the wall of a museum, and rightly so, but I ride my 1956 Mercian on the road, in the woods, in the rain, wherever need, whim, or friends lead me. Although the front hub is Airlite, the rim is a Torelli Master - but I sleep o.k., because we aren't talking here about rebinding one of Shakespeare's Quartos. If it was the first Mercian Bill Betton made for himself, perhaps that would be a different matter - but I bet if Bill Betton has that bike, he rides it.