Since the bijuu are actual people, I don't think it would be fair to whoever they are to be forced into fighting as some joined force against whatever. Unless, the plot does something that effects them individually that would actually make sense for them to be involved. I don't think they should directly affect the plot, but just be considered plot assisting characters. I think things should still focus mainly around the Yakan and Fushoo, whatever is going on with them? The bijuu can show up and do whatever when they feel like it since its actual people playing them. If other people are unhappy with the fact that they aren't showing themselves, then maybe they should try harder to search for them by getting themselves more involved with other characters.