"OK, but a Clojure purist would probably see those occasions as maybe highlighting gaps in that language’s coverage."
A Clojure "purist" is probably still programming Scheme. In other words, "purist" and "Clojure" do not go well together. As Rich Hickey often says, he is a "practitioner." He wants to Get Things Done, and Clojure reflects that. Dropping down into Java is not "cheating," it's one of the reasons Clojure was implemented on the JVM in the first place, and I've seen Rich recommend calling Java code to solve a problem multiple times on the Clojure group.
Amen. I was irked by the "purity" factor as well. Adam's (ato's) response in the comments is dead on: "I needed an Atomic counter. Java has one. I used it."
One of the biggest draws of Clojure is that it runs on the JVM so you get access to Java's libraries. It is baffling to me how someone could describe using Java via Clojure as being a no-no.
A Clojure "purist" is probably still programming Scheme. In other words, "purist" and "Clojure" do not go well together. As Rich Hickey often says, he is a "practitioner." He wants to Get Things Done, and Clojure reflects that. Dropping down into Java is not "cheating," it's one of the reasons Clojure was implemented on the JVM in the first place, and I've seen Rich recommend calling Java code to solve a problem multiple times on the Clojure group.