It would be nice to allow variables to be marked to say that (eg) they must be defined before a build can run. This way the variables can be defined at a high level, and redefined at a lower level. However, right now you don't know whether you have defined all the required variables at the lower level. I can see this being particularly useful for builds that are supposed to be manually triggered, and require values entered at that time. e.g. perhaps a repository label to use for a manual build. When the build runs (manually), any variables that have been defined as requiring a value, and don't currently have a value, the user would be prompted for on the manual trigger page. If it's a scheduled build, and variables are missing, then the build would fail immediately, which will mean that troubleshooting should be easier.