在Symfony2上，使用Controller返回另一个Controller的非响应值是不好的做法吗？

I believe that the flow of Symfony2 is the following : Request -> Controller -> Response.

Actually, with this code, MessageController is not returning a response Object, but only a value for another Controller :

MessageController of Compagny/MessageBundle :

<?php
class MessageController extends Controller
{
    public function getAction()
    {
        $message = "I display this message";
        return $message; // It's not a new Response object, it returns only a value
    }
}

InterfaceController of Compagny/InterfaceBundle :

<?php
class InterfaceController extends Controller
{
    public function indexAction()
    {
        $data = $this->forward('MessageBundle:Message:get');
        $response = new Response($data);
        return $response; // This one return a Response object
    }
}

Is this code a bad practice ?

Yes. Because if you decide later to use Message::get directly, you won't be able to do it or fix it without changing InterfaceController.

I suggest this:

class MessageController extends Controller
{
    public function getAction()
    {
        $message = "I display this message";
        return $this->createResponse($message);
    }
}
class InterfaceController extends Controller
{
    public function indexAction()
    {
        $forwardedResponse = $this->forward('MessageBundle:Message:get');
        $response = new Response($forwardedResponse->getContent());
        return $response; // This one return a Response object
    }
}