Is it within the EPUB 3.0 specification to have a pagebreak span without an id?
<span epub:type="pagebreak" title="2"></span>
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Pagebreaks are usually used in conjunction with nav document page-list sections and since you need an id to refer to pagebreak markers in the book it'd be rather pointless to use pagebreaks without ids and a corresponding page-list section.
You can find an IDPF-approved sample epub3 book with pagebreaks and a page-list nav section here.