Killing a party member only helps the game once. In my first game of 7, Aeris was a primary party member. In my second game and all games thereafter, I didn't bother with her - it wasn't worth the effort. Thus, when Aeris died, it didn't just kill her for the duration of that game, but permanently. I can't be bothered to put effort into characters that I know won't last the game. This also applies to games like FF2 and 4, where temporary characters come and go untill the very end. I won't bother with anyone who won't be in my final party.