This window is a result of this thread on the EQ2 forums:
You can test the affect of having no outline on the chat window text. This window has a small button in the top right that will toggle the chat text outline on and off. By default the outline is off. If you use something like FRAPS or ATI Tray Tools you can see the difference in frame rate between having an outline and not having an outline.
Here are some screenshots.
If you use the normal black background there isn't any reason to have a black outline because the text looks exactly the same without it.
If you ever use your chat window with a transparent background the chat text would be harder to read without an outline.
A black outline makes the text more legible if it's in front of a light color. But, this outline requires 6 times as many triangles to render.
My video card is so low-end that the difference in frame rate doesn't seem to be very much, even for a window of text stretched from the bottom to the top of the screen. I'm curious to see how much difference it makes in other people's frame rates though. If the difference is significant maybe SOE could make the outline optional or have no outline by default.