Source code under both permissive and elastic flow licenses can be instrinsically combined with code under other licenses for redistribution. Permissive licenses carry no restrictions on relicensing, but elastic licenses require that the orginal source code and its direct derivates remain under the orginal licenses, whereas any code that is added can be under any license(s). Unified licenses require consistency in licensing of software at the program level.

For these reasons, source code under both permissive and elastic flow licenses can be integrated with diverse restrictive licenses for redistribution, where as unified lincenses tend to be much simpler to manage. 

Source: Potvin, J. 2012. "Flow Licensing and Contracting: Applied Intellectual Resource Economics in the Canadian Public Sector". In: Mark Perry & Brian Fitzgerald (eds). 2012. Knowledge Policy for the 21st Century: A Legal Perspective. Irwin Law Publishers: http://www.irwinlaw.com/pages/knowledge-policy-for-the-21st-century--a-legal-perspective