The functionality of logging users in to access their data from a mobile app for example is more a case for OAuth. 
You can read more about OAuth in Magento here and about the configuration here.

The API's are more meant to exchange information not tied to a specific user.

Extending Magento API with customer login
A while back I was reading through various question/answers on Stack&nbsp;Overflow. I stumbled upon one question (related to web technologies, maybe REST?) that had a beautiful and humorous response that started in normal text but slowly degraded into amazing Unicode characters and approximations of normal English text using various symbols.  Where can I find this post?  

I wanted to point it out to someone.
