Mercy's ult still charges pretty fast, so if I'm confident that that one guy who just died might be the only one if I rez him up and push the fight heavily in our favour, I will rez that one guy.
But its really a big bet because if he fucks up as soon as he's back or your team still fail despite basically having +1 player, then by the time your ult is charged you will have lost a push completely, that you could have maybe turned by rezing a team wipe. Playing Mercy really requires to be aware of the momentum of both teams and playing poker.