Digital BW, The Print

The Epson driver removes all CMY from 100%K, there's no way around it. I suspect the Piezo driver does too. The Epson 
driver begins replacing CMY values with K ink some where around 80%, and has removed it all by 100%.
The only way to print any cmy at all at 100%K is with a RIP.
It's commonly referred to as a "rich" black.
Generally not lighter than 100%K only, but depends on the specific ink, paper, and percentages.
It's usefullness varys.
Tyler