Google's Pierre Far addresses a common misconception regarding the use of rel-alternate-hreflang on this Google+. He said that you can use rel-alternate-hreflang for the URLs that are multinational sites, spread across top-level domains.
The usage is applicable even if the URLs have the same languages. Far also cited a few examples including:
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 SwitzerlandMultinational Sites over Top-level Domains can use hreflang!,