I would like to argue that it would be nice to have this.
The same exact things above can be said for when you're editing a comment "Simply navigate away from the page" and it won't be saved... but we do have a "cancel" button for editing a comment.
It seems that, for consistency, it would be nice if the cancel button were always there.
Personally, I often hit the "add a comment" link accidentally instead of the "edit" link and the system doesn't seem to mind that both are open concurrently but it still seems odd to me that the "edit" function has a "cancel" link but the "add" does not.
If I'm not finished with the page, being able to close the text area would be nice. And, clearly, the link is already in the code somewhere, as it's already active for edits.