And why would someone want to cruise on a real fast boat ???: Because to some cruise on a slow boat is boring :-) A slow boat will take you there but for many cruising is not only the pleasure of voyaging but also the pleasure of sailing and in what regards that a slow boat is a boring boat, one that you want to sail on autopilot.
Even on a fast one sometimes, with weak winds, it can be boring but when the wind gets up to 10k and over you have a sport's machine in your hands. Sailboats are slow if you compare them with cars, a car is slow if compared with an airplane but you will have much more vivid sensations in a car than in an airplane and the same can happen in a fast sailboat. The sensations are not in accordance with its relative slowness. Between sailboats 1K difference can be a lot and you can think that is nothing but that is not the main difference between sailing a fast or a slow boat. More important is the sensation and precision you have at the wheel, the fun you can have sailing it ....and anyway that extra knot will mean three more days on an Atlantic crossing and that is something ;-).
Of course, with the boat I am talking about, this one: http://youtu.be/qu3DpS8qeA8 you will go way faster than 1k comparing with a slow cruiser of the same size and when you get on the trade winds with 25K you will sail easily at 2 digit speeds and while surfing waves slightly sideways you will have the same sensation that you will experience at the wheel of a powerful sports car going fast on a twisting dirt road, controlling the slides. Off course some would not like to do that in a car either and will found that very disagreeable too on a sailboat and that's why there are some that prefer to sail in slow heavy boats.
For the ones that like fast ones this was a great idea: Alexis Guillaume, founder and instructor at the "Sail away" sailing school decided to give sail lessons in a different way and have fun while doing it. He picked up a Jumbo 40class racer with a cruising set up (a boat very similar to the one of Eric) and started cruising across the Atlantic, to South Africa, Brazil, Caribbean and back to Europe, taking as crew "students" and making some major races along the way.
You can see some nice videos here:
and have more information here: