Here's a case I play devil's advocate. I actually quite like Xion. And to add fuel to the BHK's fanbase's firey tears, I never cared about Roxas in KH2. ¯\_(ツ)_/¯
To me, he was flat. He had all the melodrama of the BBS trio (of which the same folks that heavily criticize this fact will typically ignore in his case) rolled into a tedious poster-slapping prologue. Days gave me reason to care about his plight, and my desire to see him live as his own identity was born from that plot. The relationship between both of his friends was beautiful and simple and more sophisticated that it gets credit for.
Many say Xion's unnecessary, but I think she just gets heat because she's a main character in that game. Admitting you don't like her just because / tastes is almost a better argument than merely saying she's unnecessary, because, tbh, there's a whole lot of nonsense floating around in the series. The same could be said of almost anyone else with good reason. "Let's create 13 Org members, because that's an unlucky number, but let's give their personalities each the depth of an -insert anime trope here-." Or "Let's make characters that literally look like Tidus, Wakka and Selphie so you don't have to think too hard about whether or not there are similarities among these people." I will say, one area Days deprives us of is the dimension of some of the other members, besides Roxas/Axel/Saïx. However, I don't think the Roxas's story is diminished because Xion is present. I think it's embellished in a way that's very believable.
Vexen's experiments brought us the Replica Program, and it's only natural that he would have birthed these experiments (which I'm glad they didn't just let die after CoM but gave more heft to), to manipulate the power of the keyblade wielder(s). One of my favorite quotes in the series comes from Re:coded's Reconnect scene: (Yen Sid to Mickey) "A man like Xehanort will have left many roads open." Xehanort is calculated af. He's going to utilize every possible option in any given timeline, and Xemnas having multiple keyblade wielders, however flawed, accelerated the goal of summoning Kingdom Hearts.
Xion herself is a prime example of the different kinds of beings we see budding hearts based off the experiences that shaped their individuality. She provides the push to Axel's pull who continues feigning content living life in the Organization (projecting it onto Roxas) while she resolves to seek out the truth.