I don't do that for two reasons:
- 1. The clutch throwout bearing will wear out very quickly ($$$) by keeping clutch pedal held to the floor at a stoplight.
2. I was told a long time ago that by having your car in gear at a red light, if someone bumps you from behind, even slightly, your foot may slip off of the clutch and you'll find yourself out in the middle of the intersection waiting to be hit.
Not sure I much I believe the second one, but I've remembered it all these years.