That's funny. I'm always trying to leave space in my pack. When I go hiking it never fails that one of my buddies has a heavy pack. We'll usually distribute our gear more evenly...why make the guy with the 60lb pack suffer and slow everybody else down. If I can put a part of a tent, some pots, or whatever in my pack and even things up, the whole trip will be easier and everybody will have more fun.

Also, It's common practice with search and rescue to leave 20% of your pack free for 'team gear'. If I show up with a drum tight pack to a search or even to a training exercise, then have to carry 100 feet of rope and some carabiners or extra first aid equipment as my share of my load, I have to ditch stuff to do it. That takes up valuable time and I might, in my haste, leave behind something important without thinking.