I've been told by an SEO company to add a
hreflang attribute to every page on a single-language website (ie. 'self-referential' tags), but everything I've read about the hreflang tag, including Google's take, suggests that it's only really useful for multi-language websites. Adding this would be a huge amount of work for this particular site, and I really can't see the potential benefits (or even any indication that such a thing is considered best practice by anyone).
This Google Webmaster blogpost does recommend using self-referential
hreflang attributes, but only in relation to multiple-language websites.
"If you have multiple language versions of a URL, each language page must identify all language versions, including itself. For example, if your site provides content in French, English, and Spanish, the Spanish version must include a rel="alternate" hreflang="x" link for itself in addition to links to the French and English versions. Similarly, the English and French versions must each include the same references to the French, English, and Spanish versions."
Is there any SEO benefit to adding such a tag to a single language website?
E.g. Just a single instance, like:
<link hreflang="en" href="http://www.example.com" rel="alternate">
...to the document
(Please note that this is in addition to the obligatory
lang="en" attribute in the