People forget vi(1) is part of POSIX so required on "systems that both support the User Portability Utilities option and define the POSIX2_CHAR_TERM symbol." [http://pubs.opengroup.org/onlinepubs/9699919799/ ]
The former is probably a good idea, seeing as the User Portability Utilities option in POSIX is written to be: "a requirement for a user portability interactive system. It is required frequently except for those systems, such as embedded realtime or dedicated application systems, that support little or no interactive time-sharing work by users or operators"
The latter is defined to mean that at least one terminal type has all user control options.
Unless Arch Linux wants to be deliberately non-POSIX compatible, vi should be in base.
The Linux kernel, glibc and various GNU utilities already deliberately deviate from POSIX compatibility in many ways. I don't think it's a very important consideration, just trivia. It's great to have vim-minimal on the install media (which is now the case), but that doesn't mean it needs to be in base.