For programmers it might feel odd, if pressing tab makes the textarea loosing the focus, instead of putting a tab character in the text.
But yeah, it sucks to write a real amount of text in a texbox that doesn't let you use tab. Of course you can always write in another editor and then put it in the textbox.
Even easier if you use something like this:https://addons.mozilla.org/en-us/firefox/addon/its-all-text/