The most likely reason for disabling extensions by default is most likely to avoid confusion in a way, and also in a UI stance, makes a file kinda look "dirty". I wouldn't want all my MP3s or Images w/ thumbnails having the extension afterward. A solution would be for Microsoft to add some detection system to tell weather or not a file has an incorrect extension, and notify the user when they go to run it. Problem solved! It can keep the cleanliness and be secure!