Hi does anyone know if quad 6870 running on 890FXA-GD70 mobo is a good idea?
Sure, depending upon your requirements. It's a bit expensive if you are starting out now.
I see most miners go for the dual card option instead of quad. Why is that?
Cooling. Cheap mobos.
By having 4x card dont you save more on other hardware cost (no need for another set of mobo/cpu/memory) as well as less electricity?
You sure do.
Is there some hidden inefficiencies in the quad card setup that makes them less ideal?
There is almost no clearance between the dual-slot cards if you have adjacent ones. You should get some PCIe cables or extenders or wedge them apart and get some fans blowing on them.