It's mostly used in dueling, for battle it's just too short.
I'd argue that it is an exceptionally well rounded weapon. The great maul is short but with coordination and the right lineup, it can be one of the best weapons to use in certain situations. The bec may be rather short but it is devastating to plate infantry. The bec allows you to use an athletics build 18/21 while maintaining the same damage or better against high armor as all of the other top tier weapons, at 27/12.
Also, basically 1/5 of 2h/pole on NA is a bec user. Very very few people in NA use a flamberge yet in the huge multi clan eu tourney, several people used them.