It another personal choice this.
I do flush the old oil, but not by the method the OP mentions which is just an extra oil change, albeit for a very short time.
I use fully synthetic 5W-30 C3 to LL04 spec and as I do a high mileage of mainly long journeys my oil change intervals are extended to 12K
I put in a 375ml can of TEC 2000 Oil Flush and run the engine for 30 minutes then drain the lot and change the filter.
If I had an engine of unknown history I would be flushing and changing the oil asap.
I'd also flush before change of oil spec.
I'm open to criticism for over spec with my choice of oil, just as with upping the spec of my tyres, but at the moment I have 5 vehicles all of them with over 120K and never had any lubrication problems.
"Oil is the cheapest mechanic you can get." my old Dad used to say.