In my mind, this usage of parenthesis is less ambiguous. Parenthesis are used in C-like languages for both function calls and order of operations. That is to say, they are used in these two ways:

    foo(a, b, c)  // functions
    (a + b) * c   // order of operations

Making parenthesis optional for function calls means parenthesis are used instead for just controlling order of operations.

In practice, CoffeeScript can't always guess when something is a function call (such as a function call without arguments), so you end up mixing in some function call usage no matter what.