FF would still be on. So, if you put an arty on barrier while your teammates are trying to plant, they die... only difference is that now, so do you.
Nothing you're saying is untrue, but I think the benefit of forcing players to watch their own friendly fire is far greater than occasionally killing a teammate when it's their fault. 99% of the time, my team damage received is far higher than my team damage given.
At a minimum... I haven't seen any arguments against FF reflected for panzer/flamethrower/mortar.