Sometimes its handy to have a tiny bike around. A small bike is easy to store, easy to park, easy to hide. If you need to take the train or a cab, you can bring it with you inside. If you find a new bike in the alley, you can throw the Pixie on back and keep riding. The Rat Patrol has quite a few of these useful little things, some more ridable than others. Generally, your knees will ache after a long expedition, and you'll never get the kind of looks a chopper gets, but the Pixie bike makes for a great ride.
Love tall bikes but are afraid of heights? Think pixies are the cutest but hate sore knees the next day? Build a small tall, all the joy of two bikes on top of each other and the cuteness of a pixie welded into one bike. Put the crank on the bottom bracket and you've got the leg extention of a full size bike.