Why curved seat tubes? Here is what I have heard:
On single bikes, many Europeans (others, too?) used to believe (still do?) that being seated over the rear wheel made a bike climb better. I doubt there is any basis to this, but I could be convinced otherwise. Maybe this belief started because many strong climbers slide back on the seat, effectively sitting over the rear wheel (but I'd think this was to get behind the BB, not on top of the rear wheel)...
On tandems, the appeal of a curved seat tube is a shorter wheelbase. With a bike this long, a shorter wheelbase improves the handling, especially on tight corners. Of course, most classic tandems are shorter already than many modern ones (in some cases by almost a foot!) Doing a U-turn on a two-lane country road illustrates the difference quite nicely. A standard 1980s Jack Taylor tandem can turn easily, while many moderns have to put a foot down. The downside of a curved seat tube is stoker comfort, with the stoker sitting almost on top of the rear wheel.
Jan Heine, Seattle (stoker comfort - makes you wonder about carrying a child in a seat attached to the rack!)