we have weekly 30-45 minute mtb races in my district that transfer to cross skills and fitness well. i plan on riding my cross bike on mtb trails too but i need as much help descending on off road stuff as possible so i dont see how starting off on a stabler bike wont help boost my confidence getting down stuff fast.
i dont look at mtb racing as getting me ready for cross as much as i see it as something to help prevent burning out from riding and racing the road bike so much.