One of the main rules of physics is that when riding a bicycle, walking on a rope, or simply standing still, the closer your center of gravity to the ground, the better.
That is, getting your body low, with the classical attack position, allows you to get your CG closer to the ground:
So, which are the components of your bike in which a weight reduction has the bigger benefits because of lowering your overall center of gravity?
Right, the more closer to the ground, the less important, as this graphic shows:
So reducing your weight in components far away from the ground, will lower your center of gravity, thus improve your balance and stability. That is, reducing weight in components like stem, handlebar or seat would have the greatest impact in terms of balance and stability.
That was the good news. Bad news is that mtb riding involves quite a bunch of physic laws, as for example that reducing the unsprung mass, that is, the weight of the components not supported by a suspension, greatly improves the handling of the bike.
But this is the topic for another post.