In fact, sort of hard to justify when Apple still doesn't use the i7 chips and lower clock speed. I suppose for me, I wonder what you can build that runs as quiet. That is worth the price, especially with audio production.
The Core i7 and the Xeon 3500 (which Apple uses) are the same bloomfield chip and use the same x58 motherboards. The Xeon version has ECC support enabled there are no other differences, price included.