Query objects are incorrectly altered by toString() method

Re-addressing an issue I first mentioned in #564852-22: Subselect with placeholders causes invalid/duplicate placeholders. It seems the SelectQuery::toString() method incorrectly modifies the called object, thus resulting in incorrect query results for more complex queries.

Attached is a (probably still) failing test case for this scenario. The error can also be reproduced with the following code:

 $query = db_select('node', 't')->fields('t', array('uid'))->condition('t.uid', 2);
$query = db_select($query, 't')->fields('t', array('uid'))->condition('t.uid', 1);
(string) $query;
print_r($query->execute()->fetchAll()); 

And regarding Crell's last comment in the other issue:
Select queries are non-reusable, unlike the other builders, for various reasons.
I don't see why this is important here. Calling toString() should hardly count as "using".

Also, can someone tell me how you can find out the real SQL query executed, with all placeholders replaced, etc.? I have again problems with a complex query and suspect it might be a problem with placeholders, but can't test it because of the unhelpful toString() method. Is there any easy way?