Not create her? As far as I can tell she has served two purposes. She motivated Changes and she's a potential source of income in a spinoff series. He could put her in boarding school and never mention her again in the main series and make better, tighter books. Which is my personal opinion and one which I'm sure isn't widely shared.
Oh, I agree with you.
I actually resent her presence a bit, it takes away from Harry's development as well as other characters' in the series that are far more interesting and were actually there already. I really don't think she was a necessary addition to the series, I think Changes could have been brought on in some other way, it didn't have to be a child.
I think she stalls the story, and the books have been going a bit downhill since Changes, imo, except Battle Ground, in which she wasn't a character really but a thought (that could have been easily replaced by something or someone else).
Besides, Harry was already doing the whole "I'm responsible for this human being" thing with Molly, who thank god grew up, because I couldn't take much more of that teenage thing, but that's personal preference. I don't like children in adult stories, they stall.
About Maggie as a character, I dunno, she's not well fleshed imo, sometimes she doesn't sound the age she's supposed to be. Zoo day was interesting only because of Mouse. But then again, I really don't think I wanna see more of her, unless she starts going psychotic or something, I don't see her as being that interesting.
Same thing with the whole Bonnie thing, which gets all mixed up, is she a daughter or not? imo not, but people consider her as such.
And now we also have Thomas' child. I dunno, seems all so repetitive. And Jim probably has some great plan, but I dunno, these are additions that I haven't particularly enjoyed because they take away from what was already there and could have been developed otherwise.