Speaking as a professional fencing instructor (a friend on MT got me to join once she found that they had a fencing subforum. Hello to everyone, by the way) footwork in fencing requires and builds a tremendous sense of balance, foot discipline, sense of distance, leg strength, lightness of feet, sense of tempo, timing, speed and acceleration.
That being said, it does not directly translate well to any sort of fist fight or other activity where there is a great deal of lateral movement (there is lateral movement in fencing, but not a great deal of it. The focus, as was mentioned above, is on forward and backward motion) or where there are methods of attack that do not involve something at least tokenly similar to a fencing weapon.
Fencing footwork, like just about everything else in fencing, especially modern Olympic style fencing, is highly specialized and while it requires many of the same qualities as another combat sport or martial art (I think my list above is a good wish list for someone developing their footwork in any sort of martial activity) it expresses those in a way that is very good for fencing, and not necessarily for other things. As such, it can be an excellent cross training tool for those wishing to develop these qualities, but should not be taken as a technical foundation.
As far as fencing's relationship to Jeet Kune Do, I'm not going to make the same tired statements that so many dabblers post on the forums at fencing.net, and I'm sure appear from time to time here. I will, however, add what I know from my own experience: An old coach of mine made The Tao of JKD required reading for me because of the fencing specific parts of it. Not things about fighting that are adapted from fencing or which could be adapted to fencing, but things describing tempo, attacks, counter attacks, distance, etc in fencing terminology. I have never studied Jeet Kune Do, and I stopped doing various eastern martial arts when they started to interfere with my fencing, so I will not say to what degree things were influenced by his exposure to fencing. I have seen people with heavy eastern martial arts backgrounds come to fencing and become much better for the cross training, again primarily because of the footwork, but also because of the keen senses of distance and tempo (both hand and foot tempo) as well as the tight timing required and encouraged by it.
To end my tangent before I start talking about things I know effectively nothing about, and continue where I DO know something, there are many different schools of thought within fencing as far as the footwork is concerned, both with the use (the proper time, distance and tactical situation) and execution (tempo, speed, form) of certain techniques. If one were to look at, say, the Polish National Team vs the Russians vs the Hungarians vs. the Germans vs. the French vs. the Italians, one would see six (or more!) distinct different sets of footwork, each of which can be very effective on an international level. One thing I have learned in my time fencing for different coaches, teaching different students and observing fencing from very bad local high school teams to olympic gold medal bouts is that there is definitely not one unified "Fencing." There isn't even a unified "Foil Fencing," "Epee Fencing," "Sabre Fencing," or even "Aladar Kogler's 18 year old students at the New York Athletic Club" fencing. Especially at a high level, each fencer and coach has their own interpretation of things, from as abstract as the proper moment to steal distance from an opponent who is trying to use their footwork to prepare an attack to how one should stand in their on guard position. Each variation has many subtle plusses and minusses that the fencers learn about and adopt or abandon over the years as they develop their own style of fencing, which tends to be influenced not only by the coaches and fencers that they train with, but also their own personality and preferred tactics.
Sorry if I wrote a book there, but that's what happens when you get fencers, especially coaches, talking about footwork.
