In this case it's justified, because there is no ambiguity. Just as a mathematician would write using traditional notation without parentheses. At the courses I

In this case it's justified, because there is no ambiguity. Just as a mathematician would write using traditional notation without parentheses. At the courses I attended in group theory, module theory etc. we were always required to prove that things are associative before dropping parentheses. Lisp notation just requires less characters (the '+' isn't repeated), but the convention is the same.