Both are invalid points because if you're riding on a road and/or in public with traffic, you need to be aware of your surroundings and scanning into a rearview mirror is quicker and allows a rider to do it more often that turning one's head. I've ridden up on many cyclists without mirrors who were completely oblivious to my presence so they had no idea what traffic was around them, they may as well have been riding with blinders on. As for pillars or passengers, yeah, my motorcycles don't have either so I suppose mirrors aren't necessary? Seriously?

Instead of making excuses for not using mirrors maybe it's best to take advantage of the tools that are available that help us riders be more aware of our surroundings... you can't argue that it's a safer way to ride.