I just checked linux mint's 17.3 manifest and lots of its packages includes both architectures. Why is that?
Why does a 64bit version of linux mint need both i386 and amd64 versions of a package?
linux;linux mint
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).