On one hand I'm inclined to agree with you. Thin is not always better. On the other hand I'm holding my 3 pound Macbook Pro comfortably and easily. It's light enough that I don't have to think about slipping it into my day bag (something I did think about with the previous iteration) and thin enough to easily grip with one hand and walk around. When I was traveling, there wasn't even a question about whether I would bring my laptop. It's just too easy not to bring.
Sure, there's a bunch of flaws in the design and I hate that they removed the SD card reader, but it's probably the closest I've seen to the platonic ideal of a laptop. Something that you can always carry on you, that you can pop open in short notice and quickly slip back into your bag when no longer needed.