My own take on this is that prevailing "sports medicine" during the classic era (1930s-70s) held that athletes shouldn't drink excessive amounts of water. I forget the medical "reason" for this, something about the build-up of fluids effecting muscles or something. These are the same people who prescribed drinking double cream for those suffering from stomach ulcers! But generally, this falls into the European/British distain for water and their amusement at Americans' insatiable appetite for what Churchill called "this bleak beverage".
All of this came into the news upon the death of Tommy Simpson in the 1967 TdF, his death attributed to dehydration execerbated by drugs and, tellingly, his drinking brandy offered by a spectator earlier in the day. Complying with prevailing wisdom, cyclists were ALLOWED four bidons of water per stage and this was what was provided at the official "refreshment zones". But even European cyclists couldn't survive on this and this is precisely why you had those wonderful "looting" raids on cafes en route, specators offering bottles of anything liquid, people with garden hoses and, of course, the main reason for "water boys" on every team. It was all absolutely against the rules and technically teams could be fined for it. But without it, Tommy Simpson wouldn't have been news.
If you look at photos of the Tour and Giro over the years you can see many cyclists with double bidons (common during the 20s-30s mounted on the handlebars) so the need for liquid (and not necessarily water, cold tea was the preferred British cyclist's drink) was there despite what the doctors were telling them.
But two sets of water bottle boses on a racing bike? Tis an Americanism like CPSC mucked up components and all those damn reflectors stuck on every surface. And the beginning of the end of the classic racing bike. These twin bosses arrived c. 1980 and within five years, the whole thing had been ruined, no?
Peter Kohler
Washington DC USA