Let me try again.
Lets say you make $100/hr editing videos (simple number for ease of calculation). Lets say you only make money when you are actually editing the work, not when waiting for it to render.
So a project on a dual-core system takes you 16 hours to finish work, but you can only charge for 8 hours. That will be $800.
The same project on a quad-core system takes you 8 hours (high-end software can really use the extra threads). You get $800 for the same project in half the time. Or you can work on TWO projects, while one is rendering, you are editing or doing sounds or whatever. You just made an extra $800.
Do you see where that system would make sense?
Again, workstation vs. home computer. If time is NOT money, expandability is NOT an issue, and you want the smaller-foot print, blah blah blah, THEN the iMac is a better deal.