Applies to modern bikes as well. I shaved 2.5lbs off my Copperhead's weight, and find it to be more nervous on any uneven surface. It's not like you feel that you'll lose control, but some of the tank-like stability has been lost indeed. Adjusting fork and tyre pressure does help a little, but it still feels different.
In general, lighter is said to be better, and there's some truth in that. it'll accelerate quicker, handle lighter and be better uphill. However for off-road riding or cobblestones, I reckon 30lbs is just about perfect. In those circumstances I'll take a 30lbs bike over a 20lbs one any time.