White LEDs also have better (and less 'harsh') colour rendering.
Coloured LEDs, while not quite monochromatic, are certainly getting on that way. An RGB led has three sharp spikes in its emission spectrum. 'White' LEDs are better, with a softer spike in the blue range, and a broad range of greens, yellows and reds emitted by the phosphor.
Of course, nobody is going to be illuminating a room with Pyra keyboard backlights (please try though, it would be spectacular) - but the white mode of RGB LEDs always looks wrong to me, somehow. RGBW would do, I suppose - but who needs the colours anyway?