Sadly, by letting the ruffians go, he's likely waived his kiri-sute gomen/uchi-sute, his samurai privilege of cutting down peasants (or those otherwise of lower social rank) who have dishonored him.
On a related note, if I remember my collegiate Japanese history course... while the tradition had much waned by the Edo period, but in the Warring States period, a married lady of the samurai class was often considered the castellan/chief retainer in her hubbie's absence/while he was out campaigning, thus it was right and proper for her to be trained in a martial art so as to better organize the defense of the home. Thus, Chiyo fits right in with her training.
(The naginata, being a polearm, was typically seen as an appropriate weapon to compensate for the typically-lesser physical strength of a lady, as was the bow.)
tl;dr: Chiyo is best wife and Soujirou lucked out big time on his arranged marriage.