... enjambment is a word that means that you're wending your way along a line of poetry, and you're walking right out to the very end of the line, way out, and it's all going fine, and you're expecting the syntax to give you a polite tap on the shoulder to wait for a moment.... But instead the syntax pokes at you and says hustle it, pumpkin, keep walking, don't rest. So naturally, because you're stepping out onto nothingness, you fall. You tumble forward, gaaaah, and you end up all discombobulated at the beginning of the next line ...
-- Nicholson Baker, The Anthologist, 2009
He's nailed in Nonnie. As I see it, enjambment is like seasoning -- a little can add intrigue to the stew, too much is...well, too much.
That's the key element of poetry, isn't it? The Line. As with prose: it's the paragraph. That distinction obliges the writer to look at form in two distinct ways. And form dictates content, which may seem a paradox, but isn't.
For me, content dictates form. Why play games with the reader, unless, of course, the reader is a student of poetry? If I'm wrong this might explain why much poetry seems to sneer at all but elitists, e.g. Ben Lerner's The Hatred of Poetry.
Enjambment can energize a poem in the most wonderful ways.
Maybe enjambment hadn't been invented yet when Yeats was writing. Imagine the energy The Second Coming might have had with some line breaks that made readers work to get what he was saying.
Enjambment done with little or no regard for readability by the uninitiated is the poet's way of saying, Let them eat cake.