From my limited knowledge: If you have the clutch depressed at any time, you are working your throw-out bearing causing wear. Also, with our hydrolic clutches, I believe resting your foot also keeps the clutch from readjusting itself as your pressure plate wears down over time.
Most of the wear and tear on a clutch happens under transition to load. Over time, your friction plate can wear down just like brake pads from matching the rotational speed of the flywheel with the rotational speed of the clutch. This can be exacerbated by slipping or riding your clutch a lot where you are in motion under throttle but not fully off of your clutch pedal. An example of having to do this in every day life would be starting on a hill where you have to come off the clutch slowly to prevent a stall. Another example is launching down the "1320" or quarter mile but I think that deserves a different thread. :wink:
For my part, I keep my foot off of the clutch whenever possible, including stoplights.
(edited for some creative spelink)