The main purpose of the fin keeled cruising boat, sometimes called a "racer/cruiser" is to emulate the high performance of the racing boats. As with everything in boats there a comprimises...as you approach the performance of a racer, more work is nessesary to handle the boat and the more uncomfortable the boat becomes in nasty weather. The "classic bluewater boat" is a heavy displacement full keel, they are easy to handle, comfortable in nasty weather, though are not super high performance. As with the fin as you approach the extremes of the design both the advantages as well as the disadvantages increase.
Very well put..