The disadvantage is that people will value "holiness" over individual people's lives. Another disadvantage is that it slows down innovation and free thought. Also, I think the main focus of people should be on humanity and living together as humans, instead of on some thing up high in the sky (or whereever it may be) of which we don't know whether it exists or not. Another disadvantage can be seen all over the world right now: religious extremism. Killing in the name of some divine being. Even though some may say it's not what religion is, it's a disadvantage of religion that some people lose their minds to it.
An advantage to some people is that they have something to live for. However, this is not to say that non-religious people have nothing to live for. Like anushka said, we just define it in a different way. And in all honesty I think that most people can find their own path if they're given the chance. Unfortunately indoctrination usually starts right from birth.