corrideat - 7:23 pm on Jul 4, 2009 (gmt 0)
Actually, color depth is a highly valuable data. Imagine that you find out that 20% of your visitors only support up to 256 colors. Then, you'd better provide a low-color version of the site.
Nowadays, however, it is quite probable that all your visitors will support at least 16-bit, but the data comes always handy.