" I don't think that changing the oil is an absolute necessity"

One item which changing the oil does is make sure the correct amount is in the system... draining and flushing the old out....and measuring what you install is the only way to know for sure...
NO book I have ever seen says it is ok to not replace the Reciever/Dryer once a system is opened up... or a refrigerant is changed... the dessicant sack is under oil... and neither the oil in a tube ... or the moisture captured by the dessicant can be removed by evacuation...
When the books even suggest putting fresh oil into the evacuation equipment they must think that having fresh dry oil in the actual AC system is relatively important...