About twenty years ago a Dutch racer told me that the problem with bar end shifters was that it was easy for another rider to reach over and whack your lever and send you onto the small ring as he was passing. Shades of the Cinzano racing team.
Perhaps in old racing bikes bar ends made more sense with half-step gearing. The levers are conveniently located, so the double shifts aren't a nuisance. With crossover gearing, the jumps between freewheel cogs are smaller and the greater precision of a down tube lever might be more important than the convenience of the lever location.
Leonard Bulger
Ann Arbor, MI