What are the direct benefits to the mtb from moto cross training?
I'd say yes. I raced motocross for 13 years and when I decided to hang it up after racing pro for two years, mountain biking came pretty easily. The jumping especially, I've never been anywhere where there was a jump that seemed too big to do (at least in public free ride parks and downhill trails). Last year I went to Whistler and jumped Crab apple hits all the way down pretty easily, the physics come natural once you're used to hucking 70-100ft pretty regularly. Scrubbing, turn downs, staying low, how to apex corners at high rates of speed, line selection, dealing with arm pump, etc etc etc. It is just a higher intensity. That being said, DH mountain biking has some elements you'll never see in motocross. Rock beds, roots, tight tree sections, add rain to these elements and you've got a different animal.
But I will say that motocross will produce injuries, and it is best learned at a young age. I started racing at age 10 and quit around 23. At 23 I was considered "old", lol. Motocross wrecks may be less frequent than DH crashes, but when they happen, they're usually much worse.