Easing the flow of traffic would relieve congestion and so extend the life of our current road system before upgrades and widening are needed. Where roads are widened there is just more space wasted by people sitting in outer lanes and so restricting the flow of traffic. I understand why it is comfortable to sit in the same lane and this would be perfectly acceptable, even encouraged, in a system where overtaking can happen on either side. Obviously dangerous practices like weaving would still be unlawful. In countries where this system is used there does not seem to be any more danger in moving across lanes. All that happens is that traffic flows better and the roads are used more efficiently.
On motorways overtaking should be allowed on either side of the vehicle infront.