I disagree. If you're on a road bike with drops you're lower down, regardless of where your hands are. Flat bars give you more height so you get a much better view over and around cars. And you also have much better control of your brakes.
I've never hit a mirror using flat bars. It's easily avoidable.
Very true. It just takes longer to get through long lines of stationary cars. I can't be bothered with that so I use drops. It depends what type of cycle commuter you are. Some people want to get a move on and others are content to wait in a traffic queue and don't even attempt to get to the front of it by riding between lines of traffic or down the inside or outside. The flat bar riders are generally in the latter, more sedate group of riders and I suppose wide bars don't matter too much to them.
As for being lower down with drops, I disagree. How low down you are when you use drops depends on many factors such as where your hands are on the bars and how far forward your bars are, and the slope of the stem.