Weight matters more if you tend to walk or bike around a lot. Carrying a 6+ lb. mobile workstation from your desk to your car and visa versa isn't much of a burden, but schlepping it around on your back for an hour or more on a regular basis can cause serious back and shoulder pain.
Ergonomics are important when you're either sitting in front of a computer or carrying it on your back for 90% of your waking hours.