8che
I think most would take issue with your statement that 'every instruction to you is optional'. Even though I understand your logic and argument your elucidation of the concept is too crude. One has to realize that ATC has the complete picture and that if they issue an instruction they do so with a better picture of the entire situation than you. Given that this is the situation you follow the instruction - you do not deviate from it unless there is an emergency. With regard to your final paragraph I think one word is enough - Tenerife.