Bill,
I agree with you entirely; a single 6mA GFI for the whole house would just not be practical.
In fact I could easily see 6mA of leakage from just a couple of aging heating elements, say a water heater or a stove-top plate. And let's not forget that more and more computer equipment is being used with filters that can easily have a couple of milliamps leakage to earth when working normally.
Even the 30mA GFIs common here now give nuisance trips in some larger installations, so the system has to be broken into sections.
It's also interesting to note the IEE's stance on over-current protection vs. earth-fault protection. For OCPD they make a big deal out of proper discrimination when selecting fuses or C/Bs, i.e. that the device nearest the fault should open first to minimize nuisance outages to the rest of the installation. But for earth-fault protection they're happy to specify a single GFI so that the whole building loses power. Go figure.
