Why is `RUSTC_BOOTSTRAP` so actively discouraged?

MusicalNinjaDad:

That means my only risk is of a deliberate breaking change in an unstable feature I've chosen to use

Or a bug in an unstable feature, which will stick around for a full stable cycle even if it's been fixed in nightly, because people are not necessarily paying attention to whether a given unstable feature remains functional in a stable release. Any given nightly feature can be broken in any given nightly, and fixed in a subsequent nightly; if that happens to be the point at which beta branches from, the feature will stay broken, and there's no particular reason to backport a fix for it.