I recall having a conversation with a friend that was angry at this kind of thing sometimes being called "Tail Call Optimization". It's a guarantee that your program won't literally crash by stack overflow if you recurse too much using tail calls; the fact that it might have performance benefits is secondary in importance. So the name can be a bit misleading.
Back then Rust's tail call story looked bad. The problem slipped my mind for years and now I'm suddenly made aware of this become keyword, and it seems like a great idea -- to make the developer's intent more explicit. Getting better target support for this is doing god's work.
As an aside, I would slightly argue against the idea that it's useful only for a narrow set of libraries. It's useful for the developer as a user of the language to be able to express things in either iterative or recursive style without worrying as much whether the latter is going to panic. Maybe if I had to deal with all the work of making such features functional in a language, I would value expressiveness a bit less!
