Like other Latin languages, Portuguese has a high number of perfect rhymes and frequent, inescapable assonances. The ease of rhyming in these languages has been envied, sometimes eyed with suspicion, by poets writing in the more obdurate English. But facile rhyme and inevitable assonance can become liabilities, handicaps to originality. With time familiar sets of rhymes grow tiresome, and free verse must have come as a great relief.
This comes from the introduction to a book of translations of Brazilian poetry published in 1972. It was written by Elizabeth Bishop, the American poet, and Emanuel Brasil, a Brazilian writer. (Incidentally, Kathleen Norris mentions this anthology in The Virgin of Bennington.) Here’s more from the intro:
Almost all of the poems in the volume are in free verse or unrhymed metrical verse, but since assonance is innate, many contemporary poets make deliberate use of it to give effects of near-rhyme, casually or in regular patterns. Brazilian poetry, even free verse, can rarely avoid melodiousness, even when the sense might seem to want to do so.
The few poems not “in free or unrhymed metrical verse” were probably sonnets by Vinicius de Moraes. Although Bishop – I bet it was her line – tinged “melodiousness” with slight contempt, it was (I suspect) to deflect attention from her lurking envy of a verbal world where “everything rhymes.”