I always viewed Days as an appendix game, such as Aragorn's love story with Arwen in Lord of the Rings. It doesn't necessarily add anything to the plot, but it's important for the character's personal life. Really, Days was there to fill in a gap in the narrative space between Kingdom Hearts 1 and 2, rather than act as plot exposition and addition such as BBS was. It didn't add to the greater plot, but it didn't really have to, considering that its sole purpose was embellishing the tragedy of Roxas.
Consider it as being like Crisis Core. Anyone who played FFVII knows exactly what's going to happen, and that the ending is inevitable. But it functions as an embellishment to an element of the story that already exists, rather than altering the whole story or bringing anything truly new to the table.
Crisis Core succeeded in adding meaning to an existing story element, while Days added a new story element and tried to make it meaningful,, which always makes it doubly hard to accomplish either. There was NO plot that needed to be filled about Roxas, his actions in KH2 make perfect sense without days. If Days had been like Crisis Core, it would have attempted to add meaning to KH2 (And KH1 and the other side games) by embellishing the existing characters. Instead, the entire Organization barely gets what could collectively be called a cameo, yet all character development done by Xion is completely undone. So in the end, Days undermined itself very effectively by not using any of its assets.
For example, if we had seen more of Roxas' day to day life, and maybe seen that he had gotten along with the people in the Organization (Much like how Zack got along fine with ShinRa) then the tragedy of his betrayal would have weighed more heavily. Similarly, in Crisis Core, Aerith was an incredibly likable character (at least to me) which made me feel sad since I knew she would end up dying. Not much effort is put into Days to try and elicit a similar response about any of the doomed characters in Days (Stop and think about it, every character besides Riku, Namine, DiZ and Mickey is doomed to die a year after the game begins)
And the thing that really got me was after I had beaten Days and looked back, and realized how cheaply the game had tried to manipulate me into feeling for Xion, and then everything sort of fell apart.
Days, like BBS, seems like a story that could have been great if it didn't feel like a few key plot points strung together, only to have the meat of those plto points never filled in. Both lead to strange stories that, to me at least, felt like there was always something being taken for granted.