A maxed out iMac would depreciate within 5 years, and any particular part would render your entire system unusable for week(s). Three years later and Apple adds a new feature that for the slightest reason requires an entire hardware upgrade because you can't upgrade components individually. Then factor in that the thin iMacs have terrible heat dissipation which means the CPU can't run at full speed without overheating.

That top of the line iMac costs four grand while a Hackintosh costs half that and gets you a top of the line graphics card and the ability to keep on par with the top of the line.