Open tandem frames have been around as long as tandem frames that have used diagonally aligned internal tubes and/or external bracing stays to yield sufficient lateral stiffness to provide good handling for the intended loads the frame will carry while still yielding enough vertical compliance for a comfortable ride.
An open frame simply uses larger diameter tubing often with more compact geometry to mitigate the need for adding an internal bracing tube or external stays. Eliminating the diagonal bracing reduces the amount of material & time needed to build and finish a frame, and therefore cost and complexity.
In many cases, eliminating the diagonal bracing tube also yields some weight savings, again bearing mind that the horizontal and vertical tubes typically need to be increased in diameter and/or thickness to off-set the the stability normally provided by horizontal bracing tubes or stays. In some cases, and for lightweight teams, production frames using diagonal bracing are sufficiently overbuilt to allow for the removal of diagonal bracing without doing anything else which does yield pure weight reduction: however, this the exception and not the rule.
As to whether an open frame would be appropriate for a given team will always come down to the size and weight of the riders, and in the case of touring, how they will carry luggage. These are details that must be provided to the frame builder who will then have to consider if he can practically achieve sufficient torsional resistant to frame flex without using some type of bracing. After all, at some point the horizontal and vertical tube dimensions needed to provide the needed frame stiffness will become too large or produce a frame that would be heavier and less comfortable than one using smaller tubing and diagonal bracing.
For production tandems, again... the frame's designer would need to be consulted and provided with the same information on rider size, weight and luggage loading to determine if the stock frame would still deliver the riding qualities that the builder intended.
As for consumer desire for wanting an open versus a more conventional frames with diagonal bracing, it's typically a combination of the aforementioned economies of construction (the potential for cost & weight savings) coupled with vanity and desire to have something that's visibly different and cutting edge. The latter tends to mitigate the potential cost savings at the high-end of the open-frame offerings, more so on entry-level open frame models where the cost-savings are used to preserve the lowest price point possible for value-conscious buyers.
Last edited by TandemGeek; 08-31-12 at 06:05 AM.