Google's Pierre Far announced and clarified on Google+ that you should use the rel-alternate-hreflang for URLs that are multinational site spread across multiple top-level domains, even if they have the same languages.
The example he gave includes:
- http://example.com/widget in English language for users worldwide
- http://example.de/widget in German language for users in Germany
- http://example.ch/widget in German language for users in Switzerland
He said most people feel they should not use the hreflang here but you should. He said "you can use rel-alternate-hreflang for these URLs."
For more on this topic see the articles below:
- Google Adds hreflang Link Element For Multilingual Pages
- More On Google Support For hreflang, Sitemaps & Geo Topics
- Google SEO: Canonical Country Pages To Primary Page
- Do You Use The hreflang Attribute? - Search Engine Roundtable
- Google Offers Rel=Alternate Tag For Translated Templates
- Google Stops Listing Burberry US Home Page - Search Engine ...
- Google's Internationalization SEO FAQ - Search Engine Roundtable
Forum discussion at Google+.