I'm not sure if you're trolling, or genuinely ignorant about the situation. Nobody is saying that 5 pounds in itself is heavy. Even a young child can lift 5 pounds without any effort. Nobody is talking about a quick 15 minute walk either, the same child could do that as well. But if you're paying a premium for a luxury product, why would you settle for something heavier...especially if you traveled frequently, and had to carry your briefcase or bag for extended periods of time?
If you keep your laptop on your desk, barely move it, or only short distances when you do...then the difference between 3 pounds and 5 pounds might be negligible or barely noticeable. But anyone who spends most of their day on the go, in and out of airports, or just has to carry their briefcase/bag all day for whatever reason.....they will DEFINITELY notice the difference. This is not because they are incapable of carrying something heavier....but why should they? To show that they are strong enough to be able to do so? That's rather silly don't you think? If we were talking about the MBP, then I could maybe see your point, with respect to people "whining" about the weight. But the MBA is an ultra-portable laptop, designed for traveling businessmen and professionals. The whole purpose of its existence is to be as light and thin as possible, while still being capable enough for its target market. It sounds like you don't understand/agree with the concept of an ultra-portable.
As far as injuries go, there are several documented cases of people suffering from muscle sprains, strains, and fatigue, as well as nerve damage and blood circulation issues. The way people grip, hold, and carry things affect their neck, back, wrist, and elbow, just to name a few. How much weight do you think it takes to cause these kinds of issues? I'm not saying that everyone who has a 5 pound laptop is going to suffer one of these problems....but they're definitely at a higher risk of doing so than someone with a 3 pound laptop...especially when carrying it for prolonged periods, and over time.
My whole point in all of this was to try and get you to understand that people have different needs based on their specific circumstances. A lot of things are situation dependent, and in this case, the weight of the laptop is relative to its intended use. Five pounds might not be heavy for a regular laptop, but definitely is for an ultra-portable.