To be honest the focus on the increase in weight is some what exaggerated. Keep in mind that the 2WD system is a technique for the ordinary rider. A standard off-road bike with a full fuel tank together with a normal built rider is easily more than 200 kg of weight and to add an extra 8 kilos should not be problematic at all.
On the other hand if you are a professional MX rider you are much more sensitive to weight – especially if the extra weight is placed far from the centre of mass. During high jumps the rider needs to adjust the angel of the bike to land in an optimal way and every extra kilo on the bike will change the characteristics and suddenly the 8 kilos could be a potential problem. At least you need some training before you get to learn the new way of riding the bike.