Well, actually maybe yes, no need to add it to core. But what this does is more than just having a live preview. You can edit AND SAVE the css files on the go -which you can't do with firebug. That's why I made it. It aint perfect I aknowledge, needs permission settings, some more options (none available right now, actually, it be nice to be able to revert to previous css versions from the same page, for instance), and might yet find some bugs, but it's quite usefull. I'm working with a designer that is in Buenos Aires, Argentina -I'm in Sevilla, España- and we speak on skype about the page design, and when she suggests a color or any other thing, I can test and save right away without leaving the page, so she can see the change inmediately.
Using ReCSS ( http://david.dojotoolkit.org/recss.html ), she can reload only the CSS and see the design changes, and what's best, she herself can make changes for me to see.