It's an option worth exploring, especially for long day treks and more remote situations where you must have a certain minimum set of gear on your person even if you lose your pack (i.e. river crossings in unforgiving terrain). In steamy jungles, soldiers will go to great lengths to avoid having a rucksack on their back since having that area clear makes a real difference in comfort level in the heat. Sometimes a ruck can't be avoided but they try.

If you are careful and discriminating you can find military style LBE that is reasonably lightweight. You can also find military LBE from which you can cut away heavy and unnecessary components. The more modern pieces use lightweight plastic (Fastex/National Molding) hardware instead of metals.

Many of the current top-flite military gear makers (Eagle Industries, Mayflower, ATS, Tactical Tailor, etc) are using 500d fabric instead of 1000d. 500d provides about 70-80% of the strength of 1000d at half the weight. These same makers are also using smarter designs that avoid needless overbuilding, excessive fabric and unnecessary features, further lightening the gear.