How unsafe is not decoupling from mains really? If say a transformer's primary and secondary insulation fails and they suddenly become de-issolated, what kind of current limiting is really there? The whole thing would become a mass of molten metal and plasma relativly quickly, and considering the relativly high energy available this is easy for even relativly extended periods of time (a few 10's of ms's with circuit breakers) 5 volt circuits only go 'bad' with a puff of smoke and a bad smell. 120 volt circuits go bad with a ball of flame and a corpse. There has to be a solid logical boundry between what is a safe 'alternative' method of coupling to mains besides a pair of coupled inductors, which is like dark ages tech nowdays and between modern semi conductor methods of voltage regulation. Even just a high valued current limit resistor on the mains side with feedback to an internal shutoff I think is more effective than a basic transformer setup. If you acount for maximum theoretical voltage and current pulses. At least on supplies less than 500ma's or so.