It helps with things like library, tool or framework selection because you end up evaluating the language itself more so than the ecosystem. I don't think many people are actually writing their applications in more than two different general purpose languages at the same time and place. Maybe you inherit some crusty code you just wrap up and re-use but you probably aren't doing new development in that old code-base at the same time.
Shell is general purpose (perhaps even more so than Java or Python), when you look at it from the perspective that programs are simply functions you call and get a result from, or which perform additional computation.
