He's saying that Rachel Dawes didn't have to be the character to do that. They could have used Gilda Dent, for example.
Rachel was an important element in the story. But Rachel wasn't the only character who could have done what she was used for.
This is true, Doc Joker

.
However, by making her less of just a catalyst/foil for Dent but also extremely important to Batman increased the tragedy, drama and tension of the story. Rachel, as in BB, still represented Bruce's hope to live a normal life (if his parents had lived Nolan's Wayne probably would've ended up marrying Rachel Dawes). He sees her as his hope for the future. By having the Joker literally take that away from Bruce was just as devestating for him as it was for Harvey. With her death, so dies Bruce's hope for a normal life and by the end of the movie the realization that Gotham will always need Batman is dawning on him.
On top of that, it also gave something more for Alfred to do. He discovers she had already realized what Bruce is doing, even if subconsciously (she is a voice for the writer to describe what is really going on in Bruce. In this case his pathological need to be Batman, even if he cannot admit it). And after her death he withholds that letter from his master, because he knows it would break Wayne's heart even more. So like every other character, Alfred has to break his moral code and what he thinks is ethically right "just this once" to accomdate the destruction the Joker is making.
Speaking of the Joker, the fact that he lied to Batman and switched the addresses, because Batman would have saved Dent over Gilda if that was his decision. But he was in love with Rachel so he goes after her, just making the Joker that much more evil and manipulative. Plus, it is nice for once to see in these movies the girl return and just not be written out essientially as another notch post (even if Bruce didn't get Rachel in BB).
And while I agree with you Maggie is a huge improvement over Katie acting-wise, for that last reason and to bridge the continuity of Bruce's love for Rachel I do wish that Holmes had played the part in this movie or Maggie had played this part in Begins. Splitting it up was unfortunate.
Either way even if her character wasn't necessary, her presence enhanced the film and made it a better movie.