I see the real question as being what to tell the still living author upon receiving that request. I would have trouble destroying something of value to posterity, but I would also be able to foresee that and would say that I didn't think I could carry out the request when asked.

I think that wanting to be honest with someone close enough to me to entrust me with that sort of instruction takes precedence over trying to reduce the practical hassles he would face before dying (since one likely consequence of saying that is that he would need to find someone more likely to carry out his instructions like an attorney).