Apple MacBook: Thin Isn't Really That Important For Most Customers
Problem is that they are well outside the realms of reality. The difference between these two machines is 0.3 mm - about the width of the hyphen in this sentence when viewed on a 24" monitor. In other words imperceptible for 100% of people who will be using a laptop. In fact I'd go as far as to say that the 4mm difference between the MacBook and MacBook Air is unnoticeable unless you have them side.
Thin isn't important at this scale on a laptop. We don't transport them in isolation, they are usually in a case or a briefcase with many other items. Below a certain limit the weight is immaterial too.
The thing that makes the MacBook so enticing is that it sports a Retina-class display in a thin and light Apple laptop. The changes in thickness, ports and keyboard are somewhat secondary to that.
In fact I'd also suggest that had Apple squeezed the display into the existing MacBook Air 11 - not unachievable given the 0.4" difference in screen diagonal - with or without the new Trackpad, the older machine would have been the better prospect for the majority of users.