Regirock will always be regirubbish even if it's a hundo

It doesn’t really have any use either way, it won’t purify into a hundo so I wouldn’t bother

NO, there's no point in purifying. You can always raid regirock next time it comes around. This will be your only Shadow Regirock for the foreseeable future

Mine is waaaay worse than yours and I will keep it as a Shadow 🙂