As Cyrille says, the people who really, really need CMYK are a limited group. Still, having CMYK is an important thing because it’s the easiest “other” colorspace to implement that still needs icc profiles. (Grayscale is actually quite hard to do correctly, and the very best algorithms to convert to grayscale are horribly patent-encumbered.)
But the real fun starts with L*a*b*. As Photoshop LAB Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace by Dan Margulis shows, the L*a*b* colorspace really allows for some powerful image editing techniques that are nearly or totally impossible in rgb.
Which is, of course, that I’m inordinately proud that Casper Boemann and me have managed to implement L*a*b* for Krita. You can paint on it, tweak channels and everything. There are limitations: the convolution filters temporarily convert the pixels to rgb and back (fixed in trunk)
and we haven’t got all the filters and dialogs in place that make it easy to use the power of L*a*b* — but that’s something everyone can help with, now the hard work has been done.