<snipped>then he will no doubt be the first to say " I was down half a billion dollars , and now I am up 2 billion dollars" and you are not going to say to him... "bro you ain't in profit until you sell bro"
Of course, I will (if getting $$ is the ultimate aim).
It only really matters if you want to use that btc as a collateral, then, yes, mark-to-market, etc, etc.