I'd have to disagree. Having road a MTB all through my teens (downhill trails and dabbled into trials riding) the skills I learn are very applicable to motorcycles. Body positioning at low speeds in tight trails help me keep the rubber side down! The sense of balance learned from riding the MTB have given me loads of confidence now that I have upgraded to two wheels and a motor. It is by no means the only way to improve riding but it is a big help IMO.
2013 KTM 350 EXC-F