Are unconverted quote records ever removed in Magento?

tl;dr: There appears to be no other out-of-box mechanism for deleting quote records which were never converted to orders.

The Mage_Sales module has a scheduled job to clean expired quotes, but this only deletes sales_flat_quote records which are marked as inactive (i.e. is_active = 0). To my knowledge, quotes are only marked as inactive when a quote converts to an order. If this is the case, then the quote table will only grow larger and larger.

Ref Mage_Sales_Model_Observer::cleanExpiredQuotes()

class Mage_Sales_Model_Observer
{
    //...

    public function cleanExpiredQuotes($schedule)
    {
        Mage::dispatchEvent('clear_expired_quotes_before', array('sales_observer' => $this));

        $lifetimes = Mage::getConfig()->getStoresConfigByPath('checkout/cart/delete_quote_after');
        foreach ($lifetimes as $storeId=>$lifetime) {
            $lifetime *= 86400;

            /** @var $quotes Mage_Sales_Model_Mysql4_Quote_Collection */
            $quotes = Mage::getModel('sales/quote')->getCollection();

            $quotes->addFieldToFilter('store_id', $storeId);
            $quotes->addFieldToFilter('updated_at', array('to'=>date("Y-m-d", time()-$lifetime)));
            $quotes->addFieldToFilter('is_active', 0);

            foreach ($this->getExpireQuotesAdditionalFilterFields() as $field => $condition) {
                $quotes->addFieldToFilter($field, $condition);
            }

            $quotes->walk('delete');
        }
        return $this;
    }

    //...
}