- CVS is simple - it's small (fits on a floppy with bsd.rd) and easy to use on exotic/slow architectures - CVS is preferable for their current contribution workflow (centralized) - CVS is treated as a tool that, currently, just works (tm)
OpenBSD's system level support for exotic platforms has more to do with drivers, compiler & tool chain support, boot code, etcetra. These do not create a jungle of ifdefs and workarounds in userspace applications. And if these platforms are broken, they are fixed instead of worked around with more jungle and weed. So support for these things does not add bugs for others.
On the contrary -- getting your software to run on a SPARC64 is more likely to expose flaws in your code.