I can't imagine doing my ride without my rear-view mirrors on my handlebar extensions, especially the left one. I glance at it all the time and I'm never surprised when a car comes up behind me. Knowing a car is coming affects my riding -- if a curve or hill is coming up, I'll sometimes slow down so the car can pass before it's unsafe or speed up and take the lane so it won't try to pass. The right mirror comes in handy on curves.
I also have a helmet mirror but I have a harder time focusing on it. However, I've noticed that it sort of extends my peripheral vision. I'll sense movement in that mirror and check it out in my extension mirror.
I honestly don't think I would commute on the routes I have to take without those mirrors.