Interesting thread.
SOPs (checklists, operations manuals)are set by the operator (who takes the rap if anything goes wrong) and the owner (who pays for the maintenance) These people's methods should be respected, however silly they may seem, if you want to borrow their precious aeroplane. And they probably have a teensy bit more experience than the average PPL. SOPs are usually developed from the Pilot's operating handbook as the minimum, with owners/operators preferences added on.
If you think you know better than these people then imagine a scenario where you are being interviewed by an insurance assessor. Do you think you can persuade them to pay out for the damage you caused if you did things against the SOPs? Can you justify your actions with "Your Honour" tacked on the end ? Would you expect an instructor to sign you out if they thought it might end up like this?
Usually it's just annoying eg those arrogant "I don't need a checklist" types who usually leave the master switch on. Even something as stupid as that can cost the operators hundreds in terms of loss of income and goodwill from the next bod who wants to use the aircraft.