Here's why:
1. With the dedicated video card in the MBP, Snow Leopard and certain other apps which will take advantage of the graphics card (e.g. Adobe CS4) will use the graphics processor to render and process. If you're a creative professional, this is a massive difference. A 10 minute process on the MacBook could take only several minutes on the MBP (with all other things being equal). It's the same reason why $2000 graphics cards exist.
2. Screen resolution. Big thing for me. That's why I'm actually waiting for the 1600x resolution screen to hit the 15" MBP. It's an inevitability, really.
3. Firewire. For lots of audio and video hardware, this is huge.
4. ExpressCard.
If you want to do pro level work, then you'll need the better machine.
4. Apparently, screen quality. I've never seen the difference, but better viewing angles, particularly in the vertical direction, would be fantastic. My plastic MacBook has horrid angles, where the top and bottom of my screen is darker than the middle due to the poor viewing angle.
One could argue that only like 0.1% of MBP users actually utilize the slot on a regular basis.
One could argue that you just made that up.
*chomp*
