BUG #6686: plpgsql Can't assign a variable with the output of a SQL Sentence which is not a SELECT

The following bug has been logged on the website:

Bug reference:      6686
Logged by:          David Carlos Manuelda
Email address:      stormbyte@gmail.com
PostgreSQL version: 9.1.4
Operating system:   Gentoo Linux
Description:=20=20=20=20=20=20=20=20

I will provide a really simple example:
Suppose we have a table
CREATE TABLE test(i INTEGER);
Let's have a value:
INSERT INTO test(i) VALUES (1);

And a function
CREATE OR REPLACE FUNCTION foo() RETURNS INTEGER AS $$
  DECLARE
    dummy INTEGER;
  BEGIN
    dummy=3D(SELECT MAX(id) FROM test); -- VALID
    dummy=3D(UPDATE test SET i=3Di+10 RETURNING i); -- NOT VALID.. WHY?
    dummy=3D(INSERT INTO test(i) VALUES (10) RETURNING i); -- NOT VALID..
WHY?
    RETURN dummy;
  END;
$$
Language 'plpgsql' VOLATILE;

I get syntax error in both commented as not valid

I think that since all queries actually returns a expected value, and since
the function is not marked as stable, there is no reason for me to block
that from happen.