No reason to avoid single-segment namespaces.

Clojure namespaces with `gen-class` map to classes with the same full name as the namespace.  This produced some concern about single-segment namespaces being equivalent to Java package-less classes and a cargo-culting avoidance of single-segment namespaces.  In practice non-`gen-class` single-segment namespaces do not appear to cause any known problems, and have begun to appear in libraries by well-respected Clojure community members, such as Brandom Bloom’s backtick [1].

[1] https://github.com/brandonbloom/backtick