I think the key idea is that its for programmers who want to create custom web services that involve user accounts.
If you want to build a website that is basically a standard CMS, then by all means use an existing CMS system.
YUMPS is for people who want to build custom web services that don't neatly fit into the standard category of CMS/BLOG/FORUM.
Perhaps a good way to understand the focus of YUMPS is for me to explain what motivated me to start coding on it.
I have started two medium sized web projects now (tfdocs and userquotes), that involve users creating content. In both cases i struggled mightily to figure out what to use as my core system for handling users, and ended up both times choosing Drupal, because of its strong API for extending the system.
While both systems got implemented -- I was very unhappy with the process of using Drupal, and my conclusion was that Drupal and other CMS systems, not matter how flexible and programmer friendly they are -- are designed for web administrators not programmers. While they may have a high degree of customizability and configurability, trying to use them as the starting point for a custom system that is not first and foremost a standard cms, is painful.
YUMPS is like a programmer's library, with which you could build a new custom CMS, forum, blog system, or similar web service.
Or put another way, YUMPS is not a competitor to DRUPAL, Wordpress, or the SMF forum system -- it's something that could be used as the foundation code for building a new version of Drupal, Wordpress, etc.