If you break both, the device could float. Floating
parts
are generally not good in the electrical world, as their
behavior is uncertain.
Assuming the switch functions correctly (and is wired correctly),
the behaviour of a device with such a switch in the off position
is pretty certain: it's identical to a device that has the
mains lead pulled out.
So such a device should worry you no more or less than one
on a shelf waiting to be put to use.
I assume that I'm missing something fundamental and that we
are talking about different things here somehow ...
Antonio
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Antonio Carlini arcarlini(a)iee.org