Readability's job is to make text easier to read, not harder. Fully justified text is harder to read and has negative effects on retention.

http://www.adamsdrafting.com/2007/05/03/justified-text-versu...

Study 1: http://www3.interscience.wiley.com/journal/120024982/abstrac...

Study 2: http://www.eric.ed.gov/ERICWebPortal/custom/portlets/recordD...

The UK government also gives this advice in a style guide:

Avoid fully-justified text - the 'ragged' right edge can particularly help people with dyslexia in keeping reading position.