Glider
Coastermaker
The wires are a full circuit, yes. Are they the best ground your system can have... NO they are not. See my post above where I calculate the ground offsets that are possible with an ungrounded system... Like you said the lands on the board are there to keep everything at the same potential... and that's a crucial factor in system stability.
Just because you can make a plastic case does not mean you should.
And what about paint, that every case has? Your calculation is way off, and makes no sense at all... CPUs use 1.5Vish, but the motherboard only recieves 3.3, 5 and 12V, the stabilisation and lowering of the voltage is done on the motherboards (by the FETs), no case needed for that...
The reason the case is held at the same potential as the ground is because 2 metal objects in close proximity, but isolated are a capacitator, which can discharge and cause voltage spikes. Please, get your facts straight...
EDIT: I missed your direct question;
YES they are the best ground you'll ever have, because they are directly connected to the PSU ground, which is the reference of it all...