Just want to promote the lattest post in w3af blog "Don't write your own web application security scanner" by Andres Riancho. Over six years ago I posted a message to w3af-develop mailing list
I'm author of Xcobra - Web Application Vulnerability Scanner (http://code.google.com/p/xcobra/). Some time ago I knew about w3af project and decided that it will be better to help in developing of more full-blown project instead of inventing bicycle.
And I still think that in most cases it is much better to contribute to exising project like w3af than write your own yeat another web application scanner.