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You're missing the point. In a binary distribution based on Gentoo, the packages would be prebuilt with a default make.conf. The user would have the option of building from source with custom CFLAGS/USE if they wish, but most users would be fine with the binary package most of the time.
It would be just like regular Gentoo, except you have the option (possibly the default) to install binary packages built with fairly conservative CFLAGS and fairly inclusive USE flags.
If you're familiar with Arch Linux, imagine more of that kind of binary/source hybrid packaging concept but based on Gentoo's ebuild tree, Portage/Paludis package management, and the rest of the Gentoo platform.