This way, corporations and business users can pay to get the latest version and features just like commercial software, while the hobbyist community still gets a relative recent and feature-full release they can use for free, has source available and is guaranteed to become fully GPL code in 3-5 years.
The traditional open source economic models (support and customization, services etc.) simply do not work for professional software with a large number of non-programmer users. A commercial license would allow these clients to pull resources together to allow software to be written, and eventually released as open source for the benefit of all - and they are the most interested in this because they don't want to get locked in.