That is quite silly. I briefly contemplated this idea for TXR Lisp, but completely rejected it. The = function is numeric comparison, like in Common Lisp. You don't want to turn that into assignment. People used to Lisp dialects where = is numeric comparison will make the mistake:

  (if (= whatever whatever-else) (you-got-burned))

Arc code written by people used to Lisp dialects where = is a pure comparison function, or Arc code converted from other Lisp dialects, has to be carefully reviewed against this.

If I put this into a Lisp dialect, I would make the code walker issue a warning whenever the value of a (= ...) assignment is used, and provide an alternative assignment operator which doesn't have that warning.