For all those whining about the loss of the optical disc drive and how, if you don't use one, you should just purchase a MacBook Air, consider that for months the same people have been suggesting that the MacBook Pro should (and will) magically shrink to the teardrop design whilst retaining the optical drive.
The point isn't purely about shrinking the laptop's case. It should be whether more people can get value out of a HDD + SDD combo ( or laptop with 2hrs more battery time or ) than out of a HDD + ODD .
The case may shrink slightly but the substantial difference between the MBP and MBA would remain.
CPU -- probably quad cores all around for MBP and MBA's stuck at slower duals. [ The MBP being a better desktop replacement than MBA ]
storage -- HDD still present. MBA stuck at a 128GB cap while MBPs drive into the 0.5-1.0 TB range. If a smaller SSD is used as a cache for the hybrid set up then MBP would also have the "instant on" features like MBA but not run of space as quickly.
graphics assistance -- The MBP 13" at this point has close to MBA class graphics ( a bit better because the HD3000 isn't underclocked like the MBA, but same infrastructure). Pairing the 13" with a discrete GPU would bring it back into parity with the reset of the MBP line up (and make it different than MBA ).
Apple doesn't have to go down to MBA height to make it "slimmer". Just going from 0.9 inches to 0.8 inches would be slimmer and slighltly lighter weight without going to MBA like "diet" on trimming off the case. Still could have all the same ports, if not more (e.g., 2 Thunderbolt ports by moving a USB port to other side. ). If a quad core + dGPU + SDD/HDD 13" MBP is more popular than a MBP 13" with an ODD drive, then Apple will probably pull the trigger.
So it isn't just "thinner" this issue. It is a better usage of what volume is there with a different combination of components.
Besides an external ODD is going to have higher diversity anyway. This Blu-ray, bus powered option
http://eshop.macsales.com/item/OWC/MRSSBD4X/ costs about as much ($97) as Apple's less flexible one
http://store.apple.com/us/product/MC684ZM/A?fnode=MTY1NDA0Nw&s=topSellers ($79)
By refusing to go to Blu-ray, Apple is choking the life out of an ODD option. Blu-ray may not turn out to dominate digital distrbution but it is certainly going to kill off DVDs. If Apple isn't going to deliver, then let the 3rd parties in.