“It doesn't save the clutch, just your leg muscles.”
“When fully engaged, or disengaged, there isn't any plate wear”
It isn’t a matter of plate wear. When I took cars to pieces for fun there was a think called the “clutch thrust bearing”. This came under load as soon as the clutch pedal was pressed (it is actually a bearing between the static part of the mechanism that operates the clutch and the pressure plate).. That’s the bit that will wear out if the clutch is left depressed:
http://www.ehow.co.uk/list_7420765_symptoms-worn-clutch-thrust-bearings.html
From a safety point of view remaining stationary in gear with the clutch depressed (not to mention the handbrake off) must present unnecessary risks anad I would be most surprised if the practice did not attract a failure in the driving test.