Originally Posted by rufus
today's steel is extremely thin. the headtube is the thickest tube on the bike, and brazing them there, or having them cast into the head tube lugs leaves the thinner downtube free from possible damage done by the torch, either by over cooking by the builder, or just the general weakening that happens to most steels after being exposed to heat.
also keeps the cables from rubbing on the frame.
If you have someone like Mondonico putting them on... I'm not worried. Turn the bars on a bike with them on the headtube, then turn the bars on one with them on the downtube... The steering is WAY more responsive on my mondonico with downtube bosses, than my torelli with headtube ones.