a) Are they potentially usable?

If the wiring is all recent, it's common to use this sort of socket for
table lamps etc switched from a wall switch off the normal lighting
circuit.

Ha ha...we found this out recently. We had rented this fantastic flat done
out very swedish in style (not ikea ...which had no centre lights
throughout. On the landlords flat inspection b4 we left...he said "You do
know that those wall switches work the rounded three pins for turning your
lamps on and off?"....oh dear.

1 whole year of coming home in the dark and fumbling about for lamp
switches. Doh!!!

Glen