I get irritated reading this! The emirates a345 would still have tailstriked if they would have taken the whole runway, they had misscalculated the loadsheet, hadnt they?
And of course, runway behind you is useless, but runway u wont use in front of u is just as useless. I will always be able to stop before V1 and to continue the takeoff after V1, no matter how much runway i have left behind.