So?
If I were to make a point, which I didn't originally, because this whole thread is tired and old, it would be that the Mac Pro case is nicer and people tend to only compare CPU/GPU/Price. If you were to truly find a comparable case made of high-quality, anodized, thick aluminum, with custom boards and hard drive mounting systems such that you didn't need a rat's nest of cables, it would be freakishly expensive.
Sure you don't need a nice case and any $50 case will do, but heck, I saw on a modding forum where someone went completely caseless. He mounted all the PC components directly to the drywall along side his desk. That's a $0 case. The price/performance ratio on a $0 case is essentially infinitely good, but that doesn't mean it's better and it's certainly not comparable.
The closest PC case I can think of having uniqueness, high quality anodized aluminum, beautiful architecture, and high attention to detail is the Thermaltake Level 10... and that puppy retails for $800. If *I* were to build a PC today, it would be in that case, no question. I like nice cases, nice materials, nice finishes, unique and thoughtful design, etc. These are things worth paying extra to me.
It's like when someone hops into a new Kia and says it's a luxury car. Sure modern Kias have incredibly good interiors for the price. The interior might even photograph the same. But hop into a real luxury can and you'll notice the difference. The stuff that looks like wood actually IS wood. The stuff that looks like polished metal really IS polished metal. Everything is nicer, thicker, stronger, better, and real. It's not necessary, but if it's in your budget, it's nice to have.