Aside from its fantastic design and music, Necron's appearance at the conclusion of Final Fantasy 9's story either goes one of two ways. It's either a nonsensical opponent that shows up out of nowhere and acts as a pointless final challenge when Kuja already served that purpose, or it's a symbolic hurdle, a symbolic entity that has been subtly hinted at throughout the game.
Which camp do you fall into? Personally, I like Necron, though I wish the boss itself had been referenced or hinted at beforehand at some point. Since death is a major theme of the game, having the final boss essentially being the personification of death itself felt appropriate and just plain awesome. Zidane and his friends are sucking up their fear and giving a big middle finger to death and choosing to live, which is bittersweet considering what happens afterwards anyway. I can see why people don't like Necron, but I personally like it a lot.