Similar to the example of the OP:

  (def walrus (agent {}))

  ; good
  (defn frobnicate [w]
    (assoc w :frobnicated true))
  (send walrus frobnicate)

  ; bad
  (defn frobnicate [w]
    (send walrus #(assoc % :frobnicated true)))
  (frobnicate walrus)

It might make sense to also have a function that executes something like (send walrus frobnicate). But IMHO code should basically never look like the bad example.