I know very little about American railroads, but I love the shortline concept. I also love the idea of a large layout based on a whole branch line. Not filled with track, but a sparse scene with a handful of sidings on the way to drop wagons off.
My dream one day is to have something such as this based on an Irish setting. It's one of the reasons I love Dugort!
Large, complex layouts in a home setting may need a team of people to operate. They can also require a team of people to actually build or (and perhaps more so) finish them. Few us have the time, space, money, or number of friends(!!) to be able to do such a project. I really enjoy reading about these type of 'system' layouts though. I suspect they are increasingly rare in Ireland and GB.
When it comes to exhibitions, @Mayner has made some excellent points. To add in my own thoughts, that's why the best shows have a variety of types and styles of layout. I'd say it's for the larger, mainline, roundy roundy layouts to keep things moving and keep folks entertained. Especially kids. For smaller layouts - cameos, micros etc - then that isn't so important, but it's the intimacy that gets people hooked. The chance to chat with an operator, or even occasionally have a go yourself, is part of the appeal. As a fan of the bucolic and 'off the beaten track', I prefer the latter, but we need a range of layouts to entertain visitors.