I've been working with the Durst + VuTek UV curable, white colorant machines since their inception. Since UV Curable can "fry" a ink splotch on just about anything (glass, metal, wood, gatorboard, etc.) this process has little to worry about as for as dot gain is concerned. More importantly, it is the non-white materials that present the need for a white ink. One company did a big campaign where they printed their logo (which contains white) on a section of a wood fence. White on wood, black gatorboard or brushed aluminum looks really cool! But people with these printers aren't using white that much due to the learning curve on behalf of the designers and the need for shops to educate them.
Printer manufacturers have long considered putting white in signage printers but held back due to the low demand and high level of complications. Durst introduced their white colorant printer as a means of entering the UV curable printer market with a splash. VuTek and others reluctantly followed suit to give Durst some competition. Printing with white is a hot and growing topic right now but the real world usage and demand isn't very high.
Bringing white to desktop ink printers with pigmented inks is considerably more difficult. In addition, the vast majority of inkjet media is some variety of white which lessons the need for white ink. Existing users wanting bright whites on image areas can print on bright white paper and print the cream (or whatever) paper color outside of the image area. Looking at it from the manufacturer's perspective, I wouldn't realistically expect this technology to reach the desktop anytime soon.