I think we have to remember that the verse line has to do with rhythm, and
often with metre also. In a prose poem you draw on other aspects of poetic
language, but give up the rhythmic patterning, well, at least up to a point.
   There can still be plenty of rhythm in prose, of course, and some people
would classify some folk tales/fairy stories as a form of poem because of
the use of patterning, like He huffed and he puffed, etc, the use of three
of everything, and symmetry of the episodies in the story.
    Also it's worth remembering that poetry has not always been written
down, and when written down not always laid out lines (Anglo-Saxon poetry is
the most obvious example).
    On the downsdie, i.e. for 'advocates' of prose poetry, there are plenty
of prose piece, short stories, which carry an emotional message very similar
to poems, some of Chehov's stories, for example. And here we could think of
the interest in narratives in contemporary poetry - as far back as whenever
Robert Lowell made his remark that poems may be very like Chehov short
stories.
   Then again, with free verse there is often a reliance on space on the
page, which may not always 'replace' heard rhythms, but is not the same of
that, and does form the basis of lineation. You often find that the
rhythmical shape of a free verse poem cuts across the lineation, and the
lineation now has a different - perhaps semantic - function to do with
highlighting words.
   It's a good idea to keep feet on the ground here - or we will get over
precious and see all sorts of emperor's clothes - and remember Michael
Hamburger's comment (in The Truth of Poetry) that really free verse is a way
of laying a short piece of writing that doesn't have any other clear
category. More generously you could see the prose poem as a form in which
you want to signal that readers are supposed to look INTO its language. But
then with Joyce's Ulysses we'd do that anyway.
      My view for what it's worth - and not original - is that free verse is
a feature of modernism which was valuable in shaking up the dominant
paradign of poetry (that it should be metrical), but that (Eliot's point) it
relies on the reader's having a sense of metre in the background, and it
keeps up some approximate allusions (rhythmically) to metrical poetry.
We've now come a long way since 1912 or so, and the freedom of free verse
has cut loose and except for the layout it comes to the same thing as prose
poetry (Some might say that my 'except for' is too big a one.
    Perhaps the really interesting question here is how far Pound's emphasis
in his best known theorising on imagery and grammar - that is the concept he
has of 'the image', and the idea of juxtaposition and suppressing
connections - have contribution to the communicational problems many readers
(including me) now have in reading poems whose field of allusion and
reference is unfamiliar. Maybe not 'elitism' but certainly a problem at a
simple 'let's read this poem' level.

      I agree that none of this matters that much as long as the text your
read rewards you. But for the poet, composing in a metrical even if
'sprung' form, makes a hell of a lot of difference, and it ain't got nothing
to do with freedom.
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