Stability mainly matters when you release a library and it stops working on a newer compiler, breaking everyone downstream. That's a pain for the downstream users, because they need not necessarily understand your library internals, and the software they're building will be completely broken until you update it.

If an optional developer tool breaks, it's only you that's affected, not downstream users. You can wait for it to be fixed, no problem.

Rustfmt uses internal Rust APIs to parse and reason about the code. It could use something else (like syntex), but that would be more work.