Good Question ! To be honest i'm no expert with the rel attribute (so someone correct me if i'm wrong) but from what i can find Google can verify an identity on a web page via the attribute. Such as
<a rel="me" href="https://plus.google.com/110037486217106671520">Luke Southam</a>
i know this is off the subject of site navigation but it shows Google is reading and indexing the attribute within the <a></a> tag. As for using it within site navigation, if it (possibly) helps Google (or other search engines) to index your website better I say go for it.
