Well they're great for storing ratchets!
Have a few more for odd size/shaped tools, clamps, etc that don't play nicely with other tools in the roll cabs.
Mine aren't close to full extension though - I lose 8" left underneath the drawer above. Not much of a problem with large tools or shallow boxed/cased stuff, or with a sticky drawer liner (like under the ratchets), but a PITA if small roll-y stuff finds its way in...
Got a 3-stack of larger ones in the basement that extend farther out, but I've decided they take up too much real estate and will be hitting the road (whenever I get around to it...)