I just can't understand it, that's all. They had their time, they're having their time again; it's, quite honestly, not fair to the new cast for them to take up their space and their screentime. Having a favorite is fine, but not to the point that it detracts from other characters. That's just blatant fanboyism. It'd be like saying: "You know what would be cool! If Zetta went around in every N1 game, kicked EVERY main character's butt, and became the new main character!"
Sure, in side-stories (usually starring Laharl instead of the much more awesome and superior Zetta - see, I have favorites too) this kind of thing can work, but if you put that right in the main story, well... It's just not that fair. Not to mention that, usually, a character gets all their growth in their game. When they're put in sequels over and over, their growth usually ends up being subtracted so their "hilarious unlikeable character quirks" can interact with new characters.
Hell, this is exactly what DLC/extra scenarios are for. If Neptunia really becomes a success enough to warrant a third game, be it a sequel or another reboot, that would be perfect for this kind of thing.
@Mizu: Sounds to me like we may get redone scenes, yeah. Maybe with the sisters instead of the goddesses?