Interop also relies heavily on reflection, and may require type hinting once it becomes a performance problem. Interop also implies the use of side-effects since we're relying on Java object. While a lot of Clojure's sequence and collection operations do work on Java sequences and Collections, it's not perfect and very easy to get bitten, requiring a Clojure.walk/postwalk and zipmap with coercion to get a good Java->Clojure container/sequence/collection.

If we used interop for everything mundane, Clojure would really just be an S-expression shaped husk over Java code. Not a very great solution imho.