Apple doesn't enter markets where it can't make money. There is at least some money to be made selling proper workstations with Xeon CPUs, ECC RAM, and workstation-class graphics cards with application certifications, etc. There are customers that are going to be looking for those requirements, and aren't going to purchase consumer-grade equipment. The new Mac Pro offers a lot of value per dollar for these customers. The customers who are happy to run workstation apps on a home-built consumer system probably have no interest in Apple anyway, except at a price point that makes no sense for Apple to sell a product at.