To oversimplify: religion leads to conflict, while spirituality leads to unity. Spirituality is individual, and at the same time more connected to the universal whole. Religion, by creating a group, necessarily creates another "us" to be contrasted with "them", as several others have mentioned. Even if a key tenet of "us" is to help "them", the division is still there. Even the Bahai's aren't exempt. Yes, there is overlap because some people do in fact find spiritual growth through a religious group, but it is not required.
This, too. A group of fervent believers is a potent weapon in the hands of a dictator, demagogue, or outright nut. A distribution of individually spiritual people? Using them would be like herding cats.