I considered S&S couplers when I ordered my first Thorn. After discussion with one of the people at Thorn I decided against them. With a full-length fork, mudguards, racks and lights a split bike makes two awkward packages.
I had a Moulton TSR that split in a similar way, but I didn't use the feature very much.
I don't do air travel, and so far I have always managed to get a full-size bike onto French trains on the rare occasions I have needed to.
For taking a bike on other transport with the least hassle possible I have Brompton folding bikes. These fold down small and fit in a bag that is easily carried on the bike. Once bagged, a Brompton looks like ordinary luggage so can go anywhere, even on urban buses. Not quite as good as a full size bike for road touring, the small wheels are worse than large wheels for use on tracks and paths, but I have used Bromptons extensively in both situations when I need to combine with other transport.