Frankly, Nicole was an excellent addition. Again, she brought a fresh perspective to the show, with her young-at-heart, Jimmy-Stewart enthusiasm and idealism. Originally, Bashir was the idealist--but all the years of experience toned it down. The show, with its increasingly dark tone, needed some idealism--some "innocence", if you will--to contrast it.
Well, thats the problem, isnt it? Even if you feel the show needed something like that, how on Earth can a character be innocent and idealistic when shes got all those lifetimes of memory?
I dont think the show really needed a new innocent and idealistic character, because there just wasnt time to explore it. Also, the tone of the show wasnt as dark as previous sessions.