In TNG "Birthright", we see (half a) bat'leth being used as a farming implement. Worf is appalled by this, but what if the thing really is a farming implement first and foremost?
Kahless is credited with forging the first sword of this sort, and of overthrowing a tyrant, i.e. leading a rebellion. Might be the peasants rebelled by turning their farming implements into makeshift weapons - and after their victory, these weapons were elevated to cult status and preserved (mostly) as is despite their deep shortcomings as actual tools of war.
Being great at defense would also be good for peasant rebellions: those succeed by the numbers, and if the peasants can keep themselves alive long enough to swarm the professional warriors opposing them, they win.
In TNG "Reunion", Worf wields a bat'leth but the more pragmatic Duras grabs a conventional broadsword. Perhaps it's the sign of an honorable warrior to use the traditional, inferior weapon, and the sign of a great warrior to triumph despite the weapon's shortcomings?
All said and done, I love the concept of the bat'leth - it's alien enough, and in its own alien terms it's also practical enough, or at least logical enough...
Timo Saloniemi