Backlighting can be a nice touch to your keyboard even if you don't have key caps designed for it.
Below is a picture of my KBT Pure with CCnG key caps.
KBT Pure with CCnG - Backlighting On by
Michael Ashby, on Flickr
Because the Signature Plastics design have thin sidewalls, the LEDs will shine through a bit on the side. This doesn't happen with thicker sidewalls, or with the key caps that came with the board.
Cherry MX Blue Switch by
Michael Ashby, on Flickr
As I understand it, the base of the key cap is translucent and then painted black. The etching process removes some of the paint and thus allows light to shine through. If you don't buy caps that have a translucent base and are etched like this, then light will not shine through the key cap to illuminate the legend.
It's a personal preference, but I prefer the glow
around the key cap more than the illuminated legends. So I'm in the process of looking for some PBT key caps to replace the default ones. These will be solid key caps and thus not illuminated through the key cap.
Hope that helps!