Nobody asked for a thick computer, what people are worried about is Apple sacrificing components for aesthetics. They DON'T have enough space, the 15" MBP has heat issues and Apple's "official" comment is that the 13" doesn't have space for a discrete GPU.
Why make the MBP thin at the expense of everything else? Those in need of a thin and light notebook have the MBA. If everything is thin, what do the power users use?
The 15" MacBook Pro doesn't have heat issues. It gets hot, yes, but heat issues implies that it gets so hot it cuts out. Every MacBook Pro revision has been notorious for heat, and the metal case design only makes the machine "seem" hotter.
They don't need the thickness of the MacBook Pro at the trackpad end if they remove the optical drive. The only reason it is that thick is because the hard drive sits there (open up the case and see for yourself). If the hard drive is moved to the optical drive place, then a tapered design would not cause any compromise.
I'd venture that that is the best selling MacBook pro. It really will turn customers away that they've had since the 12" Power Book. They drop the 13" I'll go back to Linux, I don't want an air.
The 13" MacBook Air v 13" MacBook Pro, performance wise there isn't anything hugely significant. The Pro will perform better, but they both have integrated GPUs. Of course, you could use the space saved from an optical drive on the 13" and use that for a dedicated GPU.
But I doubt Apple will though. They'll see the 13" Pro and 13" Air as far too similar if the 13" Pro adopted the new tapered design. In a perfect world, I'd love a 13" Pro, tapered design packed with high-end GPU and CPU. But this is Apple.. and they'll force you to get an Air if you want a 13" laptop.
And to be honest, the high-end 13" Air is a very capable laptop.
Why the tapered design? It limits the components too much and causes even more heating problems.
Why not leave it at the same size w/o optical drive. This way, it could get a bigger logic board (housing the SSD memory), bigger battery and even a better heat transfer. This would lower the limitations of the internal components (and therefore maybe give the 13" a dGPU again). I'd say the tapered design will limit hardware components too much. I'd rather have a bigger sized MBP with a lot of capabilities than an ultraportable semi-pro playbook...
It could be as thick as it is right now at the top end (which is thicker than the Air), and taper down like the Air near the trackpad. The logic board on the Pro is tiny, and they don't need to make it larger for better performance. Your main concern is heat, but if you were to actually look at where the CPU and GPU is in a tapered design (keeping the Pro at the same thickness it is at the thickest point near the hinge) then you'll not be creating any more heat issues.
Apple said that the 13" Air design (tapered) was the future of notebooks. There's been rumours of a 15" laptop circulating Apple with an Air-like tapered design. I'd place a high stake bet that the next generation MacBook Pro design will be tapered, exactly like the Air, and that they'll drop the 13" from the Pro range.