The economics of using OS X for VFX work just doesn't add up. When the machines from Apple are easily double what a machine built by an integrator will cost. Add to that the hardware support has always been flaky (see: Nvidia/Apple spout causing new cards not to work).

When your house requires you to have a large influx of contact workers, you want to have the cheapest most efficient setup possible. Apple just doesn't cut the mustard here.