I use compression sacks for two reasons: 1) I am small and packs that fit me simply are small. Compression sacks allow me to fit all gear inside and not have to hang stuff off the outside. 2) Weight distribution in your pack is just as important as total weight. Poorly distributed weight feels much heavier. Compression sacks keep the center of gravity of the pack closer to your back -thus less of a "lever arm" factor that pulls you backwards. As for weight - I use the light REI or Granite Gear ones - they really do not weigh much more than regular sacks. I use one for my sleeping bag and one for clothing.