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:

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 $$
DECLARE
dummy INTEGER;
BEGIN
dummy=(SELECT MAX(id) FROM test); -- VALID
dummy=(UPDATE test SET i=i+10 RETURNING i); -- NOT VALID.. WHY?
dummy=(INSERT INTO test(i) VALUES (10) RETURNING i); -- NOT VALID..
WHY?
RETURN dummy;
END;
$$
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.