The ROI for battery life falls off at a certain point, right? For phones, it's probably about a day -- how often is it a problem to plug in your phone at some point in a 24-hour period? -- and for laptops it's often about a full workday, 8-10 hours. I'm not saying that a 20-hour laptop battery isn't an incredible accomplishment, but I do think that I care a lot less about 20 vs 10 hours than I do about 10 vs 5.

And then sit at a desk all day within reach of a power socket

This. The amount of times I've truly needed more then a couple of hours of battery life are rare. I think most people think they want more battery life when they really don't need it. Just add more cooling to stop those processors from throttling all the time.