That is not the only reason that I consider 'unity in diversity' as a highly flawed Baha'i' concept: the fact that some religions are atheistic whilst others are theistic imposes different outlooks and practices upon the adherents that are wholly incompatible with each other.
There is no reason that an atheistic Buddhist could not find common ground with a Theistic Christian. Its an inherent part of multiculturalism that works well in my country. We don't all need to have the same faith or even worldview to be able to get along.