Obviously, it depends on what you want from your old bike. My goal is to recreate the experience of the old days. To try and become a rider from the 1940s for a day.
I feel that the experience of a bike is only complete when it is period correct. So while sometimes modern components might work better, they would spoil the experience. Yes, a Cambio Corsa bike doesn't shift as quickly as many others, but that is part of the fun.
Period correctness, in my opinion, includes sizing of the bike, rider's clothing, etc. A few things, like tires, often are impossible to get correct. Fortunately, replicating the awful roads of the 1940s is rather easy these days in the U.S. - any potholed road around here will do!
Most of all, I found that most bikes are very rideable even when period-correct. Despite what advertisements want to tell us, engineering was very well-developed as early as the 1930s. Ball bearings were as good as they are now. Various steels and aluminum alloys were available and used.
Progress often is associated with changing demands on a machine. What I am trying to say is that where we see shortcomings of the old machines, it often is because we have different needs than the riders had back then. For example, brakes have become a lot better, because traffic density (and better car brakes) have made shorter stopping distances necessary. (Although the 1950s Mafac Racer, set up well, is more than adequate even in today's traffic.)
Another example: as fixed gears fell from favor, riders lost the ability to ride at a great variety of cadences, so more gears and better shifting mechanisms became necessary. In the days of the Cambio Corsa, I doubt people shifted more than every 5 or 10 km. For short hills, they probably just powered up them like they did on their fixed gears only a few years previously.
In this context, it is interesting to note that cyclotourists, who did not have the strength and training of racers, adopted gears much earlier. Thus the attitude of the racers for a long time that gears were for sissies... (to which the cyclotourists replied that their times up the tallest climbs were much faster than even the world champion's!)
You could even argue that Campy cranks seem to have broken so often in the U.S. than in Europe, because here people use their racing bikes to run errands and commute. Europeans ride their city bike (or the bus/tramway/subway) to work, and keep their nice bike for the weekend. Starting from a standstill puts the greatest stresses on a bike, and it just doesn't happen that often in a race. (Plus, many American riders are/were heavier and less smooth than the average Italian racer.) (The same applies to horizontal dropouts - you rarely dislodge a wheel in a race, but it happens easily when starting from a light.)
To conclude: I vote for period correct whenever practical. Does that mean that I'd spend $ 500 to get the correct logo shifters for my 1957 racer? No - that doesn't change the way the bike feels. If I could afford it, I might, but my family needs to eat!
And I do wear a helmet, even on the oldest of bikes. It may look out of place, but once again, my family depends on me!
Jan Heine, Seattle
P.S.: Don't misunderstand the above: There certainly were faulty designs, as there are now. But most of the stuff worked at least adequately, otherwise people wouldn't have bought it. And most of us put less miles on our classics than they would have seen in their prime, so problems like breaking cranks and slipping shift levers are less pronounced.