I like Timo's explanation that the bat'leth was some farm implement used by serfs (presumably with no access to weapons) to overthrow an ancient Klingon overlord. It's more of a cultural reminder of this, than it is of being a practical weapon.
Klingons keep them around the same way that US Marines are given Mameluke swords or sabers. It's not for practical use (they have disruptors for that), but just for show.
And also Klingons get in duels specifically of the bat'leth on bat'leth variety. Duels like Duras v. Worf, where only one wielder has a bat'leth are very rare and not advised.