I don't know about the plot stuff but one of the reasons why I would like to see Rarilight in this show is that Rarity and Twilight's developing relationship is one of my favorite things in this storyline. They have one of the most best written friendships in Tamers I've watched in fiction.
They play well off of each other, they remain themselves when around each other, they're funny together, and they're really wholesome.
While I do like Twixie both in canon and in this show Trixie is kind of abusive towards Twilight.
Twilight and Rarity are the only members of the mane 6 who have never physically attacked their own friends unprovoked.