Actually Palutena got the exact same thread.
http://smashboards.com/threads/kid-icarus-rep-necessary.358482/
As far as Rosalina goes I don't think she was necessary. She's only appeared in a small handful of games in a series that dishes out 4-5 games every year and is also the oldest Nintendo series. Yes, she represents the galaxy games, yes she brings something new to the table, and yes Mario was due another rep. But looking at other contenders:
Toad: Been in the series for about 20 years and was playable multiple times
Bowser Junior: Since 2002 has been in over 20 roles, is the secondary antagonist to Bowser, and can also represent Galaxy
Waluigi: Uhh... admittedly I don't think he's more important than Rosalina but he had a much stronger fanbase
Paper Mario: Has the entire paper mechanic and could represent the Paper Mario side of the series
All, except maybe one have resumes that exceed hers so she wasn't necessary from an importance standpoint and from a popularity standpoint she was probably the 5th most requested. One has to assume that the main reason she was chosen was to represent the Galaxy games and the fact that she was more unique than Bowser Junior. Honestly though, no Mario newcomer would've been necessary because there was always an acceptable alternate.