While backporting a new PipeWire release earlier this year, I discovered that it had added dependencies on new Meson features that didn't exist in Debian Stable. User-defined functions would have allowed me to shim those features on the older Meson version, greatly simplifying my work and minimizing my patches' footprint.
I don't know if that situation is common enough to justify user-defined functions to Meson's author, but it was frustrating, and it seemed ironic that a build system designed to solve platform compatibility problems had in some sense become one.