Rewrite a non existing magento core class....

rewrite non existing file

I found it is possible to do a rewrite on a core class that does not actually exist....

Usage case:

I am creating a reCaptcha module that integrates google recaptcha into magento's core captcha system.
In the core captcha you have the ability to state a 'type', as can be seen in the config.xml 
of Mage_Captcha

<default>
......
<admin>
            <captcha>
                <type>zend</type>   <<<<<<<<<<<<<<< specify a captcha type (model)
                <enable>0</enable>
                <font>linlibertine</font>
                <mode>after_fail</mode>
                <forms>backend_forgotpassword</forms>
                <failed_attempts_login>3</failed_attempts_login>
                <failed_attempts_ip>1000</failed_attempts_ip>
                <timeout>7</timeout>
                <length>4-5</length>
                <symbols>ABCDEFGHJKMnpqrstuvwxyz23456789</symbols>
                <case_sensitive>0</case_sensitive>
                <always_for>
                    <backend_forgotpassword>1</backend_forgotpassword>
                </always_for>
            </captcha>
        </admin>
......
</default>

so in my module I added a new type called 'recapcha' 

however, the developer of that core module kinda locked the model location down to exist 
within the Mage_Captcha namespace, as can be seen in the helper class of that module

/**
     * Get Captcha
     *
     * @param string $formId
     * @return Mage_Captcha_Model_Interface
     */
    public function getCaptcha($formId)
    {
        if (!array_key_exists($formId, $this->_captcha)) {
            $type = $this->getConfigNode('type');
            $this->_captcha[$formId] = Mage::getModel('captcha/' . $type, array('formId' => $formId));
        }
        return $this->_captcha[$formId];
    }

so Mage_Captcha_Model_Recaptcha does not actually exist, which means I cannot use this....

BUT, if, I place a rewrite directive in my module for that class/file, to a model class 
in my module, I can intercept the model instantiation, and successfully instantiate my class.

So, is this a bug, or a feature? Should I not depend on this ability?