Obviously they are very useful. I still don't think they should exist in this usage of yaml.

Once you allow setting and reading of variables in a configuration file, you lose the safety that makes the format useful. You might as well be using a bash script at that point.

You already can set and read variables. The `matrix` section is often used to test against multiple versions of software. Environment variables can be referenced. And the project configuration supports both secrets and variables configured at the project level.