Assonance, also known as "slant rhyme," is a repetition of vowel sounds that creates an illusion of rhyming. Wikipedia notes that it's "used in (mainly modern) English poetry."
Which leads me to believe that from Shakespeare's times and up to, say, the end of the 19th Century, the concept was frowned on by purists. Correct me if I'm wrong, but I don't think that Keats or Byron ever used it.
I realize that in different regions of the English-speaking world different things are pronounced differently. That said, my ear (accustomed to the way certain vowels and consonants are pronounced along the Eastern Seaboard) tells me that "defiance" and "lions" do not rhyme.
If, for instance, you wrote a twenty-page-long poem; and if one of the poem's purposes was to imitate, or stylize, early 19th Century poetry; and if that poem consisted entirely of iambic lines; and if all those lines rhymed; and if all those rhymes were perfect or near-perfect - "defiance" and "lions" would stick out like an infected thumb.
Am I wrong?