So why is it bad?
Well, /bin/vi is a very stripped down version of vim (by design), and fails
miserably to parse most directives in vimrc files.
Worse: while I haven't looked at the exact reason why, if I run /bin/vi and
it's able to load my /usr/share/vim/vim61/macros/vimrc file (which should not
be there, it should be in /etc/vim, see bug 70809), it fails to parse it correctly
but in the process it actually clobbers the file I was editing and saves the
empty version (I'm not kidding)

I've edited my /bin/vi binary so that it tries to load ~/.virc and
/usr/share/vim/vim61/macros/virc, which allows for having a minimal config file
that /bin/vi can parse

Ideally, when you build /bin/vi, you should have it read a different config file
than /usr/bin/vim