For the same reason you can buy a cadillac over a chevrolet. Bigger, better built, luxury etc...
Bigger screen (if you want it) and best resolution
Alu case - the luxury touch that has all that pro look
FW800 and express slot
GPU and backlit keyboard (as you mention) but also more processor speed, more ram and hd on stock order (sure, you can max out a macbook ; you can also order leather seats on a chevy !)
Attention to the details (look how the ports are circled)
a little bit thinner
And an iSight with a green light masked in the case
Some people won't even think about a cadillac, others won't settle for less.
I, myself, think a chevrolet will do the job very well for me
It depends of what you want and what you need
*cough-cough*
don't forget that the MBP iSight has double the res. of the MB.
Also, the MBP has an option for matte sreens if you prefer those.
Advantages that the MB has over the MBP:
Cheaper
Smaller foorprint
Longer battery
Longer AirPort range (this is only marginally longer since they put that rubber thing in the MBP hinge)
MagLatch and different keyboard (if you like these)
Smaller poweradapter
Anything else about the MBP is better.