A while ago I wondered why not many writers wrote both poetry and prose, and why few if any of them published books combining poems and prose. Then it dawned on me that many so-called "Poetry" books actually contained Flash, vignettes or micro-texts. Think of Hugo Williams, parts of Lachlan Mackinnon's "Small Hours", "Citizen" by Claudia Rankine, etc. Some books are an assortment of several categories, classified as poetry for marketing/Award reasons or because of historical inertia.
I suspect that people whose work spans a broad spectrum don't care much about where in the range a particular piece is, until submission time. Readers care most when the advertized category doesn't match their assessment. Roughly I'd suggest to reader-friendly writers that -
- If the line-breaks (like any other features/words) are doing little or nothing, leave them out, especially if there's a risk that they might look like an attempt to divert attention from weak content
- If the context might make readers skim over a text that would reward careful reading, it might be worth adding line-breaks as a hint that a different reading strategy is recommended. In this situation a common ploy is to make each stanza into a similarly sized rectangle to show that the particular positioning of the line-breaks doesn't much matter.
Tania Hershman's a particularly interesting case. She's almost simultaneously published two books - a poetry book "Terms and Conditions" (Nine Arches Press) and a prose book "Some of us glow more than others" (Unthank Books). Reading the two books together it's not always clear why a text should have been printed in one book rather than another. Indeed, a text called "What is it that fills us" appears in both, the poetry version being a slightly shorter version of the prose version.
Reviewers have applauded the genre-free approach (and the science/literature, mainstream/avant-garde mix) of these books. It's interesting to see that despite the challenge that this diversity might pose for readers, there's an emerging consensus about which pieces in "Some of us glow more than others" are best.
One day maybe, books won't be classified as "Poetry" or "Prose" but as "short texts". To do so now will reduce the chance of being reviewed and will reduce sales. "Poetry" is currently the safest option if there's a wide variety of content.