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I think most of the reason for not having all these runtime checks in core is performance.

Though I guess if there was an easy way to turn them off for production it would maybe be okay.

Curiously, the language's features for doing runtime checks as a user--core.spec and pre & post conditions--are both very easy to completely disable for production. I don't know why they're not used in core: probably a combination of the core team's feeling that they don't need them, and not wanting to make the language slower than necessary by default.

Maybe pre/post was added later?

They're now in the process of speccing all of core. Hopefully next release will have it.

I do think slow by default is part of it. I've seen a lot of builds deployed to prod without prod optimizations in my experience. That's why I think spec in not instrumented by default. Too worried people will keep it on at all times, and then Clojure would get a bad rep for being slow.

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