I could see it being useful to have the option available for those who might be slightly more comfortable reading vertically, but I think that right-to-left vertical script could be a much lower priority than getting left-to-right vertical text working.
(According to Omniglot, "In Taiwan [Chinese] is often written vertically, while in China and Singapore it is usually written horizontally.", so it might make sense to have the Taiwanese variant on the Mandarin Chinese Wikipedia default to vertical if that's possible at some point.)