T.S. Eliot's Criticism

I do not minimize the services of modern poets in exploiting the possibilities of rhymeless verse. They prove the strength of a Movement, the utility of a Theory. What neither Blake nor Arnold could do alone is being done in our time. 'Blank verse' is the only accepted rhymeless verse in English--the inevitable iambic pentameter. The English ear is (or was) more sensitive to the music of the verse and less dependent upon the recurrence of identical sounds in this metre than in any other. There is no campaign against rhyme. But it is possible that excessive devotion to rhyme has thickened the modern ear. The rejection of rhyme is not a leap at facility; on the contrary, it imposes a much severer strain upon the language. When the comforting echo of rhyme is removed, success or failure in the choice of words, in the sentence structure, in the order, is at once more apparent. Rhyme  removed, the poet is at once held up to the standards of prose. Rhyme removed, much ethereal music leaps up from the word, music which has hitherto chirped unnoticed in the expanse of prose. Any rhyme forbidden, many Shagpats were unwigged. 

And this liberation from rhyme might be as well a liberation of rhyme. Freed from its exacting task of supporting lame verse, it could be applied with greater effect where it is most needed. There are often passages in an unrhymed poem where rhyme is wanted for some special effect, for a sudden tightening-up, for a cumulative insistence, or for an abrupt change of mood. But formal rhymed verse will certainly not lose its place. We only need the coming of a Satirist--no man of genius is rarer--to prove that the heroic couplet has lost none of its edge since Dryden and Pope laid it down. As for the sonnet I am not so sure. But the decay of intricate formal patterns has nothing to do with the advent of vers libre. It had set in long before. Only in a closely-knit and homogeneous society, where many men are at work on the same problems, such a society as those which produced the Greek chorus, the Elizabethan lyric, and the Troubadour canzone, will the development of such forms ever be carried to perfection. And as for vers libre, we conclude that it is not defined by absence of pattern or absence of rhyme, for other verse is without these; that it is not defined by non-existence of metre, since even the worst verse can be scanned; and we conclude that the division between Conservative Verse and vers libre does not exist, for there is only good verse, bad verse, and chaos.

From 'Reflections on Vers Libre (1917) T.S.Eliot

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