In contrast to this kind of poetry, free-form (often known as "modern") poetry can sound flabby and too easy to write. However, the same could be said for free-form prose. Some of the above rules for poetical forms can easily be applied to prose, though ideas can come from elsewhere too. The OuLiPo group of French authors often borrow formal patterns from such other domains as mathematics, logic or chess. Raymond Queneau experimented with many such forms. Perec wrote a 5000 word palindrome "ca ne va pas san dire," and his lipogrammatic novel "La Disparition" lacked the letter 'E'. England's Brooke-Rose's wrote "Between" which avoids all forms of the verb "to be".
Prose formalists often try make the form relevant to the content, sometimes going so far as to make the form part of the plot. They might use a syllable-per-sentence sequence of 9...8...7... etc for a rocket launch, or not use the letter B when describing someone allergic to bee-stings. Sometimes the twist of the story is in the form rather than the content. It's this integration of form and content which particularly appeals to me.
So don't let prose go the way of poetry. Start fighting the flab today!