Using a webdav client is fine. The Drools Eclipse plugin (Guvnor perspective) also uses the Guvnor WebDav servlet. Just note that there is not much if at all development going into Guvnor's webdav servlet so you will most likely not see many new features or any existing bugs fixed quickly, whereas Guvnor's rest api is being worked on very actively.
>> A part from the REST API why isn't there a builtin API int he JBPM framework? <<
I don't really get your question. jBPM-console uses the Guvnor rest api to communicate with Guvnor, it is just simple Java code. Not sure we need a custom api just to do simple java.net stuff :)