Y ppl mess up the yt tags? Y? Y???
Because the way the bbcode tag works is illogical, not to mention cumbersome, and there's no clear reason why it shouldn't work with the entire URL.
Funnily enough I actually tested it in preview and it worked, but in hindsight it must've loaded the video from cache, I guess.
To be honest, simply putting on the entire actual link would be a more logical way for the bbcode to work, because it would go with the established way of adding content on posts. If you put an image on a post, you can't just put the filename of the image... you need the whole URL.
Besides, separating the video tag from the url is actually more effort-consuming operation than just copy-pasting the whole link into the tags because you have to select the identifier alone, then copy that, and paste it on the tags; by contrast, selecting the entire URL is quite a bit faster. Try it out yourself, it's way faster to copy entire URL from address bar rather than select a defined part of it from a mess of characters.
At the very least I would expect it to work both ways - both with the entire URL and with the video identifier code alone.