Euripides has often been criticized for his use of choruses, mainly because they rarely affect the action of the play. This is definitely true of the Chorus in The Bacchae, which is pretty interesting because the play is named after them. These ladies have followed Dionysus all the way from Asia to help spread his religion, but end up doing very little to help out. OK, they do make an attempt once in a while. For example, when King Pentheus shows up talking lots of junk about Dionysus, the Chorus tells him he'd better stop with the blasphemy or he'll be in trouble. Of course, Pentheus just ignores them. You can't blame the Chorus too much though; the King doesn't listen to anybody else either.
The Chorus seems to spend most of to time singing praises to their god, Dionysus. These hymns don't affect the plot, but they do give the play a sense of ritual. Here's a typical passage:
My love is in the mountains
Limp upon the ground he
Sinks. The revel races,
Vested in his fawn skin, he
Hunts the goat and kill it…
Ecstasy the raw
Flesh…And to mountains
Of Phrygia, of Lydia
He rushes. He is Bromius [Dionysus]. (5)
Though the passage above in no way affects the plot of the play, it does help highlight the theme of man and the natural world. It also carries through the motif of hunting that is threaded throughout the play and gives a real sense of the wild ecstasy of Dionysus's rituals. It seems like Euripides just wasn't interested in using his choruses to further the plot of his plays and instead chose to use them for other purposes entirely. Euripides was a rebel in many ways. His unique use of choruses is just one of many examples of how he walked his own dramaturgical path. As he got older, choruses became less and less necessary to the plots of his plays. Some scholars theorize that if he'd had his way, he would gotten rid of them all together. This drastic step would have been sacrilege in the Athenian dramatic competitions, though.