I've been a professional driver for years, it's irritating when people come to an almost stop to go around a parked car that they can clearly get past and it also impedes traffic flow.
Being able to fold the mirrors is just to stop oncoming cars clipping them - sometimes you get angry drivers (usually taxi's or delivery vans) veering towards you to "teach you a lesson" for impinging on their side of the road.
If people were more aware of their surroundings and the available space traffic would flow easier. It's about give and take and sharing the road space.
I would never go round a car if it caused an oncoming car to swerve out of the way etc.