Think Arsenal need a player as good as Giroud. Not necessarily better and certainly not worse; they don't need a Falcao or Cavani who will come in and take all of Giroud's minutes from him, because Giroud serves the team far more than is evident in his goal account.
It's like Bayern going for Lewa. Why do they need him, when Mandu scores, pressed from the front, fights for every ball, and does everything asked of him? Is he individually a better player than the top strikers like Ibra or RVP? No but he fits the Bayern ethos so much and he's doing so well, they don't need anyone else.
Giroud could be a better finisher. But look how well he links up with the three behind him. Look how useful he is at defending set pieces (Torres tee hee). Look how fantastic his first touch around the box is. Look how gorgeous he is.
What Arsenal need is a player of comparative quality who just offers something different.
They should get Dzeko in January, if City would allow it (which I doubt). His contract is ending soon, so City gotta sell soon though.