You mean an alert even if you're not watching the thread? Or would this apply to threads you watch, where you would only get an alert if they replied directly to your post or quoted one of your posts, but you would not get an alert if someone just replies to the thread, but not in reply to you?
I think this should be in addition to thread watching. If someone replies directly to your post or quotes your post, then you get an alert for that, regardless of thread "watching" status.