You got it - weight v performance. The warmth to weight ratio of down has always been superior to synthetic, but I think hydrophobic down has narrowed the capability gap between synthetic and down in damp climates. My rucksack dry weight was 10kg last year (including ruck sack) and I am whittling that down to 7.5kg (including rucksack) through cutting out the extraneous kit (whilst remaining warm / safe) and looking for items of kit that do more than one job.