There are multiple product lines. Those people who need extreme lightness should not choose the Macbook Pro line then, which should remain focused on functionality first.
I think most pros doing particularly heavy processing nowadays have a desktop or server to offload to in addition to their laptops and performance has improved enough for pros with light to moderate processing so that higher portability is better value for most people.
Nobody is saying Apple shouldn't have thin and light options.
Apple's prices and hardware designs made the 15" MacBook Pro a popular desktop replacement. For the same price as a 13" MBP and a 21.5" iMac, you could get a 15" MBP that would outperform the iMac in some cases.
Mobility is functionality, which is why things like battery life and working keys matter.
