Color support is important, but 256 has served me well for decades. Can't think of a single program that would benefit from more colors. Unless, of course, you're trying to watch ASCII-videos.
- Themes, its nice to have the same color scheme across your programs. You can always redefine your 16 base colors in the terminal and map these to syntax in your terminal program (e.g. vi), but it's cleaner to have the rgb colors directly in your theme file, especially if you switch themes (I don't do it too often, but I like to change colors every month or so to keep it looking fresh)
- If you code CSS, it would be nice to have preview of #336699 colors in your editor. To implement that, you need real color support.
- And, less important, for those who play nethack-style games it's probably nice.