jordanjay29 At posting time. It's related to parsing. Basically what happens is that the MediaEmbed plugin says "this span of text (the URL) is mine and nobody else touches it" so the Autolink plugin stays well clear of it because that MediaEmbed guy sounded a bit intense and he might be crazy. You end up with this XML:
<YOUTUBE id="123" url="http://youtu.be/123">http://youtu.be/123</YOUTUBE>
YOUTUBE elements are rendered as an
iframe. If you disable the MediaEmbed plugin, there's no template configured for
YOUTUBE elements anymore so its content is displayed as-is. That behaviour is inherited from XSLT, although you can see the same behaviour in DOM with custom elements.
I've thought about it and I'm going to relax the MediaEmbed plugin's greediness to allow other plugins to use the same text. You'll end up with that XML:
<YOUTUBE id="123" url="http://youtu.be/123"><URL url="http://youtu.be/123">http://youtu.be/123</URL></YOUTUBE>
YOUTUBE element has a template, the whole thing will get replaced but if it doesn't, the
URL element that's inside will be rendered instead and you'll get a link.
Anyway, the main takeaway is that I agree that there should be a link if the MediaEmbed plugin is disabled and I'm going to look into it in the near future.