You drop more weight just going to the bathroom in the morning. I find it extremely hard to believe that a few pounds makes a difference to any physically capable adult.
It does. If the laptop is light enough, you can handle it (grab it, put it back on the table etc.) comfortably with one hand even while laying away from it etc. It becomes almost as comfortable to quickly grab as a tablet or a phone.
Laptops are not designed to be handled with one hand. It's a nice benefit but it definitely should not be the target, especially at the cost of better usability, which is the entire issue here.
The difference between a laptop I can comfortably have in a backpack or messenger bag when cycling is huge - it goes from something I took when I was specifically planning to use it (and worried about back pain even then) to something I essentially have on me at all times now.
It's not about physical capability. Obviously people aren't getting exhausted moving around 5lb laptops instead of 3lb ones. It's about comfort and ease of manipulation.
Why is giving up a better keyword and battery worth the few ounces? How is that more comfortable overall? Do you hold it with one hand more often than you type on it?