BBS volume 2....let it be known that this is the first game in the series that I don't actually want. Not saying it's unnecessary to the story, but everyone loses interest at some point. =/
At this point I don't think any game's unnecessary to the plot. If only because eventually, Nomura'll find some way to use it (like Xion explains why there're Riku Memories in Sora's Journal, and most likely stuff in 3D, and Namine basing basically half the series and counting
In that regard I think BBS went out of its way to explain things in the games we already had but that too is a framework for future games. In that sense I think BBS2 differs from it, though, and is likely to be part of the "post KH2" games even if only in part of it. Which means it'll be framework, again - for future installments.
tl;dr it's a game in the series, it'll be referenced in the future of the series.