Exactly my point. Common (and clearly 'accepted') though that is (at least, since split upstairs/downstairs lighting circuits came into vogue), I would say that it is only fractionally less potentially dangerous (particularly for the 'DIY meddler' you cited) than having two different phases on the same plate.I would say over 99% of houses have a switch plate fed from two circuits (single phase). There was a time when a note was placed inside the switch box, saying something about the accessory being fed from more than one circuit, and to isolate disconnect more than one fuse or circuit breaker.
Kind Regards, John