I'm gonna try to reply as best as I can to the answers about this extension, giving at the same time some hints about its usage and future plans.
Why use Write Area?
It all depends on where you want to use it.
If you are using a CMS and you can control it then probably you have already installed a WYSIWYG editor of your choice, and it's tuned up exactly to work with your style sheet, your templates, images etc... Then Write Area is not for you.
On the other hand, if you are using something like this Blogger, you have only two choices: use a plain textarea or use the editor that they have choosed to implement. Writing HTML in a textarea is quite boring and even if you are used to it you can make mistakes. But if you try to use their editor and want to write something as simple as <select> (as I did in a previous post), then you might find suddendly that all of your content after that "tag" has been lost!!
I'm not kidding, it's so ugly that from that point I went inmediately to the preferences pane and disabled that editor. Writing again the content is so unpleasant that I can't believe how other people have dealed with that.
My reaction was that I needed a solution for that problem, and using FCKeditor combined with the Firefox extension system looked like a nice idea. From that point I've worked in order to integrate it, fix some minor issues due to the way the scripts run inside chrome and package it so other people might use it.
Also, even if the Blogger post editor didn't have such horrible bug, it still lacks so many things like header formatting that it looks almost like a toy to someone used to work with a powerful editor.
So besides Blogger, what other uses does it have?
The implementation doesn't have anything specific to Blogger (for the moment), it enables you to easily write HTML in any textarea (for example there are many systems where you can write certain HTML tags in the comments, but they don't provide any nice editor for that). As the extension improves it will be adjusted so it can adapt better to the different situations, now you just have to try it and don't try things too complex where it's clear that only a subset of HTML is accepted.
What about Forums?
Most of the forums out there won't accept HTML at all due to the problems trying to clean it all and don't suffer from any kind of attack, so you can't use Write Area on them. They accept what's know as BBCode, and although there's a plugin for FCKeditor that enables the use of BBCode it isn't still full developed so I've choosed not to include it until I can know for sure that it will handle properly the most commong situations (fancy BBCode extensions probably will never be handled, but I'll try to cover the common functions in most forums before including it).
Does it fully replace the Blogger post editor?
Sorry, for the moment there's one big issue remaining: upload of images. Write Area can help you to give the proper formatting for your text and images, you can include images as long as you know the URL of a web server where they are hosted, but it lacks the hooks to upload a picture from your computer to your account. I'll try to work on this issue before releasing the next version as that's from my point of view the only issue that forces me to switch between the Blogger system and Write Area.