Leaving it in D with your foot on the brake at traffic lights is fine.
When you your foot is on brake and you are in D you should be able to feel/hear a slight click, this is the gear 'disengaging'.
When you take your foot off of the brake the gear should re-engage and the car gradually starts to creep forward.
Isn't that click is to disengage the transmission lock? where once you have your foot on the brakes, you are able to shift from D to R.
On a flat road and the car on D, when taking your feet off the brakes, you would feel a small jolt, as if the clutch suddenly engaged. This would make you believe that is the clutch suddenly engaging. But an alternative test shows that the transmission is actually engaging. You do that by shifting from D to N and back when your foot is on the brakes. You will see the engine's RPM increasing ever so slightly because in D, it takes slightly more load on the engine due to the clutch on the bite point.
I could understand why they put the 1st gear clutch on the bite point instead of it being fully disengaged. This is so that when the car is ready to go, instead of re-calculating where the bite point is (I presume by increasing oil flow to the clutch until the oil pressure just starts to increase), it is right at the point where it could just start rolling the car forward. It is kind of like how a human would operate a clutch, where when on a stop light, instead of pressing the clutch all the way down and trying to figure where the bite point is when light turns green, one would step on the clutch just enough so the car stops rolling forward. On long stops, people even slip the clutch a little so the car would creep forward, so we know our foot is at that point where all we have to do is start easing up on the pedal and the car will move right away.