Let’s say I have a Safari window and an iChat window open side-by-side, with no overlap. I think the Safari window is active, but it’s actually the iChat window. I type Cmd-T to open a new tab in Safari, but instead, I get the Font Panel in iChat.
The active window is an essential concept in the Macintosh user experience. The behavioral distinctions between an active and background window are important enough that they should also be significantly distinctive visually.