No, its because originally the bards were the spell weavers. Their power was political because they would tell news to the common folk and could run royalty through the dirt if they were scorned. In fact, kings treated bards extremely well becaused they feared the power of their song/poems. The poetic spell originates from the european bards. Witches would use songs/poems to sway the spirits and deities to see things their way, as the bard swayed the minds of town folk.
In other areas of the world, rhyme in magick doesn't exist. Music and song is strong all over the world in its ties to magick because of its ability to put us into a trance/deep meditative state. Rhythm does that.
So rhyming is entirely optional although it does make spoken spells flow like a song, creating a rhythm, which helps to facilitate our focus and concentration. But so can repeating the same line or two over and over again.