Aeris was the most amazing character in the entire game. After her death everyone can see that Sephiroth (and his minions) are completely evil. The driving force to destroy this evil makes everyone want to finish the game and wipe that knowing smirk off of Sephiroth's effing face.

And she's really not useless in the game, my party really suffered after she left. Her limit breaks are priceless for their ability to heal the party and her skill with materia is unmatched. She's also sexy.