An atom would do that much more efficiently that a ref. Refs are there for more complex types of mutation (like mutating more than one persistent DS transactionally).

Also, B-trees are a mutable data structure. "Immutable" B-trees aren't B-trees. Clojure's own persistent DS tries are quite efficient, but not nearly as scalable as a concurrent B-Tree, which easily supports lots of concurrent writes. AFAIK, there are no persistent data-structures that efficiently scale writes.