Here is something interesting to educate the people who don't understand why people need as much power as possible.
http://themovieblog.com/2009/some-crazy-transformers-2-technical-facts/
Look at the rendering times - that is why they have computers connected together to do the render collectively. Those computers count in thousands of cores (I think Framestore for example has aroung 5k CPUs in their render farm)
So, now back to mac pro on smaller scale.
12 core cuts the rendering time vs 6 core almost in half. So, if you render your project for 3 days on a 6 core but only 1.5 days on 12 core then you would understand why there is NEVER ENOUGH power. That is why Mac Pro is needed. iMac is a toy for consumers and prosumers but not professionals especially not when it comes to rendering etc.
So, those of you ignorant about these facts, could you please educate yourself so you don't have to insult us with your lack of knowledge?