I just checked linux mint's 17.3 manifest and lots of its packages includes both architectures. Why is that?
All the packages which are installed for both architectures are libraries or support packages (e.g. the GTK+ engines). The
amd64 libraries and support packages are necessary for the rest of the system to work. The
i386 libraries are installed so that older or 32-bit-only software can be easily installed; examples include Skype and 32-bit Wine (to run 32-bit Windows applications or games, which is still a common use-case if you're running Windows programs).