Haven't looked at it, but I assume the use-case is opposite of your suggestion, i.e. CUE wants to offer some kind of plugin integration and embedding a WASM runtime is the safest way.
It's also to enable you to do non-CUE things in CUE or do things more efficiently (in the long run). Think of things like topological sort or processing a value recursively. Places where an imperative system is so much easier to do something.
This is WASM in CUE, to unlock more possibilities while still preserving the language semantics.
You can already do CUE via WASM, this is how the playground works.