Resignation letters by far are the most important pieces never read. I mean, this has to be the most useful and useless piece of writing ever conceived. If I were going to resign from a job, my first course of action would be to personally tell my boss (adviser, leader, or whatever cliché name he/she is called). Then the envelope of doom is handed to he/she, revealing in no particular order what exactly is my purpose of acquiescence. So if the letter is never read and serves only the purpose of explaining what was personally told before, why agonize of the letter? Mainly for me it is the last piece of my being to leave behind.
Bosses come and go, but personnel files live for seemingly ever. Thus when my next great opportunity comes along and they call for my references, my resignation letter, a shortened epitome of my life lived in servitude is explained to the person who answers the call and tries to make it sound as though they knew me.