I currently ride a Linear Sonic. The long chain through two plastic tubes is a continual source of irritation (noisy, problems with chain tension at either end of the shifting range, etc). However, when I test rode an HP Velo Street Machine GT, I noticed that they also use this system, although they added an idler system, eliminating the tension problem.
Most bents use a single chain. However Cannondale uses the same system they use on their tandems, where they have one chain for the front rider, and then a conventional length chain with a second bottom bracket on the rear. This allows the use of a standard chain in the rear and should, I suspect lead to simpler shifting (I also have a tandem Cannondale that uses this system, and it works well).
Other than the weight and expense of multiple bottom brackets, is there some reason other manufacturers don't do this on recumbents?
Mike