Yeah, I'm not even going to think about poetry with complex line indents or concrete shapes. Let them use bitmaps!
But even in work like Plath's, whether a sentence crosses a gap or runs straight through or breaks in the middle is really crucial. We don't really see that kind of "chunking" of meaning anywhere else, except perhaps advertising and the like.
I'll have a go with any other converters I can find. Might rethink the Kindle and get a tablet though.