I have found the ZK Java Web framework very easy because of its built in support for Ajax, so why should I use JSF? I have found JSF's Ajax i.e. A4J not as good as ZK's Ajax.
The answer to this is actually: It depends. There is no silver bullet for this issue.
JSF 2 comes with built-in AJAX support. Alternatively, if you want rich GUI with AJAX support, PrimeFaces, ICEFaces and others have rich capabilities built-in.
In essence, your view layer will mostly depend on the GUI, but in the Service Layer, one don't need to reference to PrimeFaces code, only Faces API.
It's all in a matter of taste: If you find ZK Framework easier to use than JSF, by all means, use it. At the end of the day, the client never cares about what Framework or architecture you've used to make your system, as long as the system does what it's supposed to do.
I hope this helps.