Good reason to go all copper.
A good reason to go with all copper pipes is to prevent rust colored water. As galvanized pipe erode they develop rust and begin to clog up. The rust is normal for galvanized pipe but does not exist in copper pipes.
Copper pipes do not contain lead
Another reason to go all copper is that copper does not contain lead.
Copper is bio-static
Copper is bio-static, meaning that copper does not promote bacteria in the water.