Regarding what this would be used for, from my personal experience with running http://www.shawshara.com inexperienced users or non tech-users tend to edit the wiki thinking that a new line will automatically translate into a line break. Some more experienced users with HTML knowledge have modified our pages and have appended <br /> to the end of the lyrics lines. Both ways are not valid. Now, I'm sure that most of you are techies so you're familiar with how the wiki works, but think about it from the point of view of a person that has never seen code.
We've been trying to make the site as user friendly as possible. To do so, we're now in the processes of converting the whole site to use Semantic Forms to allow users to easily edit a page's contents using a form instead (ex:http://www.shawshara.com/wiki/Ya_Rayah click edit with form). With the introduction of these forms we've learned that the way to go for us is not to ask users to enter two new lines, but instead to handle this conversion ourselves. At any rate, the Poem extension ( http://www.mediawiki.org/wiki/Extension:Poem) really does the trick for us.