In my opinion, choosing a bike is all about balance. A superlight bike is no good if it isn't tough enough for the job. Manufacturers can use all sorts of sneaky tricks to bring the weight down at the expense of quality - flimsy grips that fall apart, fragile and undersized tyres, shallow rims lacking in stiffness and strength etc.
Over in the road cycling world, we have seen bike weights start to creep up as riders realise the importance of aerodynamics. I think that similar principles apply to mountain bikes. Often you'll be faster on a slightly heavier full suspension frame than on a lightweight hardtail, because you're not wasting energy in keeping the bike stable or losing time over technical sections. The advent of 29ers shows that the lightest solution isn't necessarily the best; Big wheels might weigh a little more, but they're worth it if they roll a lot better. A dropper seatpost might add weight, but it's a worthwhile price to pay if you can avoid compromising your saddle height and gain confidence downhill.