see here's the thing. (time for my road vs mtb training rant) and yes of course pro mtb riders ride mainly road, but they have heaps of time, coaches and structured programs, gyms etc. Us (me) average guys have got to go for dirt quality with the time we have.
on the road it's a sub 8kg bike, low rolling resistance, and we're just sitting there waiting to bunny hop something but it never comes ... so there's no upper body and then you draft & spin along; and as we know when we hit the dirt, it's 11kgs or so, wind in your face, upper body indeed whole body goes into mad mode to cope with the frequently changing up and down terrain. So training for intensity and I say mtb intensity is key. Of course base endurance ks can be done most efficiently on the roadie, but inevitably the mtb requirement of: intensity, economy, power, tech skills - yes all the stuff Joe Friel talks about including 'specificity' i.e. matching your training to the event comes into play.
So picture your year like a large funnel at the beginning you go around the rim, occasionally dipping down and up as you approach your target event/s, as the year progresses you spin quicker and closer to the bottom. If you can do it in the dirt, then I reckon that's the main reason to train for these events.