As much as the side games don't add to the story, they provide so much for us as well as the development team. They allow us to know the stories that main games skirt over (I know there was a pretty high demand for a game featuring Roxas's time in Org XIII). Most importantly, they allow the developers to sort of experiment with new systems and determine fun ways to play. Had a bunch of the side games been condensed, sure, we would have spent less money, but the games we get would be much more primitive than what we have now. People enjoyed the command deck system from BBS, and Re:Coded and DDD allowed the team to expand on it, figure out ways to tweak it to make it better. With even more practice, I hope for them to find a way to combine the original system of KH 1 and 2 with the command deck system.
So, in conclusion, the side games allow us to go without a huge drought of KH games (as 3 would still take the same time to develop with or without them), but also keeps the development team sharp with practice and fresh ideas
Not to mention voice actors get a lot more practice with their roles, which is also nice