Doctrine_Query::getCountQuery() produces subquery if the query contains joins

Is there a reason why Doctrine_Query::getCountQuery() produces subquery only if there is joins or having clause.

Its not good for high performance environment like ours: it makes the temp file disk full and could kill the server performance.

Let's the developer the freedom to decide what the count method should return: I prefer it as option like that:

public function count($withSubquery = false)

But I want to know the reason why subquery.