It really depends on your needs. As many others in this thread have said, if it's meeting your needs, why spend the extra money?
When I bought a PowerBook instead of an iBook, I did so because the specs were drastically different. I spent a lot more money, but the type of work I do required a higher-end machine.
When I decided to sell my PowerBook and upgrade to an Intel Mac, I was faced with a different situation. The MacBook Pro specs were better than the MacBook, but not in areas that affected me. I didn't plan on doing any intense video editing or 3D gaming, so the superior video capabilities weren't necessary. I also had no intention of upgrading to 3GB of RAM, or using the express slot.
So, I opted to buy the high-end MacBook. I maxed out the Memory (2GB), and eventually upgraded the Hard Drive (7200 RPM 100GB). As a result, I have a fantastic development machine that meets all of my needs.
Sorry for the long explanation. Just figured I'd share my own experiences.