I figured I'd chime in with a use case where this presents a problem. We use git repositories and for each repository we have three mainline permanent branches that we set up builds for. We use the git clone reference feature to "share" common code: so that the "development" branch pulls the full clone and fetch since it is the most commonly updated; to speed up clones and take up significantly less disk space the two less frequently updated branches for each repo point at the "development" workspace as the clone reference (and wipe their workspace every build to clone against the reference again).
But setting up the jobs to point to the "development" branch job's workspace directory is difficult because the structures are different between master and slave. I ultimately defined custom ENV variables on the master and overrode them on the slaves: $JOBS_DIR and $WORKSPACE_SUFFIX, so that the reference path could be defined as something like: "$JOBS_DIR/job-name-development$WORKSPACE_SUFFIX". Otherwise, I was trying to use $WORKSPACE and relative paths, and I could only choose the build it correctly for my master node or my slave nodes, but not both.