People need to be responsible and aware of their surroundings. It wouldn't be prudent to engage in a long conversation while at a stop light but sending a quick response should be acceptable since some lights require you to be stopped for several minutes. If we trust someone to operate a motor vehicle we should trust them to demonstrate good judgment about when to use their phones.
I think that is okay,but how do you police it? The fact is it can cause harm on the road, its better that they develop a system where you don't have to text and drive at the same time. Its easy to just ban it all around, and not worry about this issue.
Texting while driving is plainly dangerous as any level of common sense can show, however, texting while at a stand still is only dangerous if the person takes their foot off the brake or somehow puts the vehicle into motion. Assuming one is capable of maintaining no motion then the vehicle is as good as parked, and texting should be fine then.
While making exceptions like this blurs the line of what is acceptable use of a cell phone in the car, there should be no reason why texting while stopped is found dangerous. The driver only has to pay attention to when the light changes. Worst case scenario is they miss the light change and the driver behind them honks the horn to get them going.