Since 2005 ZK has been renowned for its’ server-centric approach, providing the ultimate in productivity gains. In addition, developers can now optionally leverage the full controllability of client-side programming. It is not about server-centric or client-centric, it is about developer-centric!
Nowadays application development is a matter of dealing with a large number of technologies, libraries, frameworks, etc. You typically
have to put together things such as Hibernate or JPA with LDAP directories, content repositories and with ZK etc.