I'm hugely ignorant (sorry :)) about the canon outside the movies (which are, afaik, the true canon), but given that were tons of Jedi and about two Sith at a time (who very, very rarely showed themselves), the possible chance for the Jedi to get involved in duels and sword-fights seems to have been pretty low (even considering that some also very rare opponents used bladed weapons that could withstand the power of the lightsabers.)
Why did the Jedi train in fencing when there seemed to be no opponent against whom to use the skill?
The more canon the answer, the better.