If Talia, Selena Kyle/Catwoman, Bane, and some villain played by Joseph Gordon-Levitt are all in the movie, they have too many characters already and the presence of Neeson as Ra's Al Ghul would just be unnecessary clutter. I know Nolan juggled multiple villains fairly well before with Ra's Al Ghul, Falcone, and Scarecrow in the first one and The Joker, Two-Face, and Maroni in the second one, but five would be overkill.