Here is Samuel Johnson, defending rhyme in his essay on Milton from Lives of the English Poets (italics Johnson's, but they seem only to mark the quoted statement):
The music of the English heroic line [iambic pentameter] strikes the ear so faintly, that it is easily lost, unless all the syllables of every line co-operate together; this co-operation can only be obtained by the preservation of every verse unmingled with another as a distinct system of sounds; and this distinctness is obtained and preserved by the artifice of rhyme. The variety of pauses, so much boasted by the lovers of blank verse, changes the measures of an English poet to the periods of the declaimer; and there are only a few happy readers of Milton who enable their audience to perceive where the lines begin or end. Blank verse, said an ingenious critic, seems to be verse only to the eye.
Of course, Johnson went on to say he didn't wish Milton had been a rhymer, since he wouldn't have wished Paradise Lost to have been a different poem. Nor am I much concerned with rhyme for now, but rather with the fact that there are only a few happy readers of Milton who enable their audience to perceive where the lines begin or end because of [t]he variety of pauses, so much boasted by the lovers of blank verse. Friends, we're talking enjambment and its obverse, the caesura, how they work in different ways in metrical and non-metrical verse, and how the dominance of non-metrical practice has obscured their function in metrical practice, even for some very sophisticated readers. I certainly won't finish tonight.
Oddly enough, my current obsession (it's not the first) with line breaks began with a review by Mark Ford in the December 2 New York Review of Books of James Tate's return to the city of white donkeys. Ford notes that the book's poems
all consist of a single, sometimes pages-long, paragraph, and the line endings are again so arbitrary as to make them almost prose poems— indeed on their first appearance in magazines and other publications some were actually printed as blocks of prose. It seems to me they work much better with the ragged right-hand margin, for it gives them that little extra bounce, and makes them feel more hybrid, impure, and unselfconscious than when presented as exhibits in the long tradition of the prose poem, with its illustrious pedigree and origins in the hallowed work of nineteenth-century French poets such as Baudelaire, Mallarmé, and Rimbaud.
That seemed so strange to me, and the passages Ford chose to quote were so delightful, that I went out and bought the book, which turns out to be, in fact, a delightful book. But reading it didn't help me understand how the ragged right margin separated the poems from the "hallowed work" of the French 19th century. It seems to me that they are prose poems. In another NYRB review (Feb 28, 2002), by Charles Simic, of Tate's Memoir of the Hawk, Simic quotes Tate:
The prose poem has its own means of seduction. For one thing, the deceptively simple packaging: the paragraph. People generally do not run for cover when they are confronted with a paragraph or two. The paragraph says to them: I won't take much of your time, and, if you don't mind my saying so, I am not known to be arcane, obtuse, precious, or high-fallutin'. Come on in.
Memoir of the Hawk has ragged right margins, too. Both books, in fact, look a lot like collections of blank verse — which they emphatically are not — and those ragged margins look a lot like line breaks. But line breaks, as Tate notes, are a signal to readers that what's coming is a Poem, by God, and lots of folks do run for cover when one of them's on the horizon. So what's going on? Perhaps the ragged margin is just the result of so many of us seeing so much unjustified prose on our computer screens in email, drafts, and discussion forums. After all, the justification algorithms in Microsoft Word produce really ugly text: maybe I'd leave it ragged, too.
While I was pondering all this, I came across Henry Gould's excellent reply to Laura Carter on lineation, and Henry and I went round in the comments, which brought me to Samuel Johnson tonight and to this perverse-seeming statement: the fact that Tate's ragged margins are not line breaks makes them, in one important respect, exactly the same as Milton's actual line breaks — neither should affect performance of the poem in which they appear.