Why is how heavy this lens is even that important? It's true that technically the Sigma is heavier. But you're talking about an item that weighs less than two pounds. That's a pretty pathetic complaint in my eyes. It weighs less than a bottle of soda or water. C'mon guys, if you spent the time you spend complaining and talking about lens weights in strength training, you wouldn't have anything to complain about. Pick up a weight or two once in a while. Lenses aren't heavy.
Try carrying 8 of them in a bag and the differences add up quickly, although I'm usually more concerned about the diameter. The fast primes (35, 50 and 85) won't fit side by side in a many messenger bags while the 24 f/2.8 IS, 28 f/2.8 IS, 35 f/2 IS and 40 will.
Exactly! When you carry multiple cameras, lenses, flashes, batteries, etc., the weight of each item has to be considered. Also, consider carrying that stuff up & down stairs, into & out of vehicles, etc. After a half day or a full day on your feet, the difference between a 10 pound bag of gear, a 20 pound bag of gear, or a 30 pound bag of gear can be very significant. And if the heavier bag results in a back or knee injury, it can be the difference between working and not working. Also, as the weight adds up, it can be the difference between being quickly mobile with a shoulder bag, or being slow to move around with a wheeled roller bag.
Back in the early 1970's when Olympus designer Maitani was designing the original OM-1, he wanted to create a camera that would weigh 600 grams, along with a system of lighter lenses. With a few lenses, a photographer could have a bag that weighed three kilograms or so — about half the weight of a typical bag of Nikon gear. More at this link: http://www.olympus-global.com/en/corc/history/lecture/lecture2/part9.html
Maitani described what this meant to one photographer: "He [photographer Don McCullin] told us that he had been able to capture his amazing battlefield photographs in Vietnam and various other war zones because his camera was light. He wanted to thank us for that. My eyes filled with tears when he told us that the OM SLRs had lifted a weight from the shoulders of photographers everywhere. He really understood the significance of our efforts to create compact, lightweight cameras. That was a wonderful moment."