Those are interesting pictures.
On the face of it, 17" sounds to be about four inches longer than 13", but the overall area difference in both displays is tremendous as your picture in the first post shows.
I think the 17" MBP is a professional level desktop replacement computer. The 13" MBP, while it share a similar name, is neither a desktop replacement nor a professional machine.
At best the 13" inch MBP is a very, very high end, sleek, sexy consumer laptop. And while it's the best consumer laptop on the market, and perhaps most expensive "entry level" laptop sporting just the standard Core 2 Duo and modest RAM and modest hard drive, it's an amazing machine on many levels.
That being said, it's not in the same league as the 17" inch MBP.
For my purposes, and budget, if my next Mac is to be a laptop, the 13" inch MBP is just perfect for me since I won't be a heavy filmmaker or hardcore gamer. It's still a little more machine than I need, but that will ensure more longevity. 🙂