I think the point is that the build.properties file should just be defaulting to the system property, encouraging the user to control the value that way (no meddling required).
It's also clearer for seasoned Ant users since they are going to look in build.properties first, and they will get what we are trying to do.
-Dan
-- Dan Allen
Senior Software Engineer, Red Hat | Author of Seam in Action Registered Linux User #231597