As you noted, VBP provides a robust GUI front-end for easily designing, debugging, and maintaining builds, and VBP also provides many built-in actions for a variety of third party tools and common build activities, is regularly updated with additional features and enhancements (NAnt hasn't changed much in years), and purchases include (responsive) support, minor updates, and bug fixes. VBP is also very cost-effective, often paying for itself in time saved within a month or two.
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