road riding is awesome for mtb riding. you make huge deposits in the pain bank and withdraw on race day
the only things road riding does not give you is:
a) target fixation training. you need to know how to not target fixate on an mtb so you can ride fast and close to rocks and trees while somewhat out of control but not actually hit them...the best training for this is actually motorized dirt biking, get used to seeing trees and crap fly past you way faster than you can pedal...then when pedaling, nothing will ever surprise you....your visual cortex and reflexes will be much sharper and things will seem like slow motion. (bonus upper body workout wrestling a dirt bike)
b) look ahead. a must for high-speed cornering. on an mtb you must trust that while looking far ahead, your brain will automatically remember what is coming under your wheel, and that you can really look ahead on an mtb and not worry because you will still navigate fine...if you train for it
c) upper body strength and the pounding your arse takes. only mtb riding can really deliver the training needed to take the beating and absorb it with minimal fatigue. working weights in a gym is pretty close, but still need real roots and rocks for full effect. so many mtb death-marches I have done in the past where pure roadies with 100 more FTP watts are simply wrecked after 35 miles and I can nuke them on the road ride home...standing high cadence on a road bike for minutes at a time can train your legs to lift your body weight off the saddle under load and this helps a ton on the trails to smooth things out and stay motoring. plain old truth the faster you go on an MTB the smoother it gets, but with standing-up high-cadence road training, even better
otherwise, road riding is insanely awesome because you can put long hard efforts in with no breaks, whereas with an MTB is it sometimes hard to do a solid 20 minutes or more at constant max load...because you often need to break cadence or stop pedaling and whatnot to navigate terrain. road riding can be pure 100% uninterrupted suffer which is good to do once a week at least
tl-dr: if all you ever did was ride road and wrestle bears then you'd be fine on an mtb
stldr; see what stonerider posted