I find if completely natural to expect to be able to assign to
arguments, and for reasons entirely unrelated to Javascript. An
argument is a binding in the environment created for evaluating the
body of a lambda expression when it is applied, and as such there is
no fundamental difference between it an a temporary variable. I see

| a |
a := 3.
a := a + 4

and

[:a | a := a + 4] value: 3

as the same thing. In this light, I see the "classic" policy of
disallowing the latter as an arbitrary restriction, even when
supported by emphatic hand-waving.

Oh, and the VisualWorks compiler has a configuration option to  allow
assignments to arguments.

--Vassili