It is important to reduce unsuspended weight because when you bounce over rocks and ruts, this weight goes ballistic, literally. On an unsuspended bike all the weight is, of course, in this category, and on a front-suspension bike like my wife's, the weight closest to the unsuspended rear is largely unsuspended weight. On her bike, as you get further toward the front of the bike, a smaller and smaller percentage of a given component's weight is unsuspended weight. Unsuspended weight is important, though not nearly as important as turning weight and rotational weight. The brakes, the skewers, and the suspension fork crown are all unsuspended weight even on a full-suspension bike.
Skewers are a cheap place to save weight.
Some people feel that titanium skewers are less rigid than steel ones, which is most important in the front, hence the combination. And rigidity is basically a good thing. Next there is high weight which is weight that is high above the ground. High weight includes first and foremost, the seat and the seat post, since these are indeed the highest part of most mountain bikes, though sometimes the stem and headset are a little higher. The importance of this weight is that as you get further above the base of the bike, a partial loss of balance of the bike becomes harder to fix.