The various swing bikes are novelties and are no doubt fun to ride for a while. I once rode a bike that was hinged at the seat tube and the head tube, so you could ride it in a Z shape. However, once you've ridden in a Z shape, and done some tight circles, it's, er... well, that's it, really.

Once you start steering with the back wheel, you may as well do the job properly and get rid of the front wheel. You then have a very capable and versatile machine but have to put up with a lot of silly comments from dog walkers.