I am going to go out on a limb, here.
I think that this all stems back to the issue in using Nvidia boards with intel.
The old 13" machines packed an intel processor with an integrated, but very powerful, Nvidia GPU (powerful for the size of the machine, anyway).
The new 11" and 13" intel i3 & i5 machines all use the crappy integrated intel GPU, whcih is substantially less powerful than the Nvidia GPU. However, in the 15" and 17" MBP's, aplpe has enough room to implement a dedicated ATI GPU.
So, instead of making 11" and 13" high-powered computers (which is now impossible because Apple cannot use Nvidia dedicated GPUs in the smaller computers), Apple is phasing out the Macbook, and soon the 13" Macbook Pro.
Thus, within 6-10 months, I can almost promise, Apple will ONLY offer an 11" MBA, a 13" MBA, a 15" MBP, and a 17" MBP. Now that the 13" MBP only has the intel 3000 HD graphics, it's really no more powerful than a 13" MBA at the vast majority of tasks. Why keep the 13" MBP around?
for those saying an 11" screen is too small for daily use, that is just ridiculous. I have used a 10.1" netbook screen for a year, and then an 11.1" netbook screen for a year. Zero problems.
I currently own the 11" MBA, along with the 13" MBP. I can tell you that in day-to-day tasks, the MBA is actually faster, because of the SSD.