There are licenced guides (usually cost higher) and unlicensed volunteer guides. By law unlicensed guide cannot get paid for their service in Japan hence volunteer and you only pay transport expense, admittion fees, etc.

Many volunteer guide are well intended, some are college students want to practice English.

Common thing about volunteer guides which concerne local touist office is that today many volunteer guides get information from internet such as Wikipedia and often information volunteer guides have is not accurate.

I am not sure that you will be exposed to more "real" Japan by using volunteer guides compare with licensed guide. Again, what voluteer guide show you and tell you can be based on personal preference/taste of that particular guide and such may not represent majority of Japanese people's taste.