I own two Macbook today, one of which is a 15" pro. True justification comes from both a technical point of view and commercial-sense point of view. Both of them have been upgraded to 4GB of RAM and both are on Mac OS X 10.5.
As far as I am concern, screen size was the first reason I contemplated a Pro. The Macbook's 13.3" just couldn't handle my concurrency theory and its non-LED type screen makes it painfully dimmer to work with at her brightest setting. I initially wanted a Mac Cinema Display but it contradicted my mobile workspace theory. Next, the keyboard. The Pro keyboard (not on the latest model though) just makes typing fun and fast. The thickness of the laptop is another factor to switch camp, giving me a far better grip and weight distribution while carrying the Pro.
The processor gain wasn't significant (between the two) as the 64-bit OS cuts through the day's work load effortlessly. Windows OS is generally still a 32-bit OS (64-bit is available but suffers from driver availability and application support). Lastly, it was the built-in DVI port that the Pro has that won me over as my days are packed with several presentations.