Just wonder if anybody here read Marchaj's book 'Seaworthiness: The Forgotten Factor' where he shows why: the most important reason is that a full keel with its bigger surface area damps the rolling motion better.
It is a very interesting book, it discusses the dynamics of sailing in great detail and points out that while a fin keel performs much better in ideal conditions (flat water), stormy weather with big seas is a whole different story.
Thank you for mentioning this book. I searched, got a look at some inside pages, and ordered one a minute ago.
One page I saw talked about the motion within a wave, and how a short keel is affected only by the motion where it is located, but a longer keel averages the motion out more, throwing the boat around less. This particularly applied to a course other than broadside to the waves. Also, I think it said that the turbulence coming off the short fin keel reduced the effectiveness of the rudder, the idea seemed to be that in flat water the fin keel worked better, but in rough conditions the long keel worked better.
Overall, the book looked a bit "dry" for pleasure reading, but I could digest at least a bit of what I read, so I think I'll learn some things from it.